Surface-emission laser diode and surface-emission laser array, optical interconnection system, optical communication system, electrophotographic system, and optical disk system

ABSTRACT

A surface-emission laser diode includes an active layer, a pair of cavity spacer layers formed at both sides of the active layer, a current confinement structure defining a current injection region into the active layer, and a pair of distributed Bragg reflectors opposing with each other across a structure formed of the active layer and the cavity spacer layers, the current confinement structure being formed by a selective oxidation process of a semiconductor layer, the pair of distributed Bragg reflectors being formed of semiconductor materials, wherein there is provided a region containing an oxide of Al and having a relatively low refractive index as compared with a surrounding region in any of the semiconductor distributed Bragg reflector or the cavity spacer layer in correspondence to a part spatially overlapping with the current injection region in a laser cavity direction.

BACKGROUND OF THE INVENTION

The present invention relates to surface-emission laser diodes andsurface-emission laser arrays, optical interconnection systems, opticalcommunication systems, electro-photographic systems, and optical disksystems.

In recent years, intensive studies are made with regard tosurface-emission laser devices (surface-emission laser diodes) thatproduce a laser beam in the direction perpendicular to the substratesurface. As compared with an edge-emission laser diode, asurface-emission laser diode has a characteristically a small activelayer volume, and hence, low threshold current for laser oscillation.Further, a surface-emission laser diode has an advantageous feature ofthe cavity structure suitable for high-speed modulation and can producehigh-quality laser beam having a circular beam cross-section. Thus, asurface-emission laser diode attracts much attention in relation to theoptical source of high-speed communication systems such as LAN or inrelation to the optical source of electrophotographic systems.

Further, a surface-emission laser diode, emitting a laser beam in thedirection perpendicularly to the substrate, can be easily integrated inthe form of high-density two-dimensional array, and application of sucha surface-emission laser array to the optical source of parallel opticalinterconnection systems, high-speed and high-resolutionelectrophotographic systems, and the like, is studied.

Currently, there are two major structures in the surface-emission laserdiode according to the construction of the current confinement structureused for confining the injected current, the one being thesurface-emission laser diode of selective oxidation type, and the otherbeing the surface-emission laser diode of hydrogen ion implantationtype. Any of these structures realizes significant decrease ofoscillation threshold current by confining the current injection regionto a specific region at the central part of the device.

The surface-emission laser diode of selective oxidation type achievesthe current confinement by using an oxide layer of Al formed byselective oxidation of a semiconductor layer containing Al and iscapable of achieving optical confinement of transverse mode in additionto the current confinement. Thereby, the laser diode can achieve theadvantageous feature of low threshold current and high efficiency ofoperation.

In the case of the surface-emission laser diode of hydrogen ionimplantation type, the current confinement structure is realized by ionimplantation of hydrogen ions in the form of high-resistance region.With this, the surface-emission laser diode of hydrogen ion implantationtype also achieves low threshold operation, similarly to the case of thelaser diode of the selective oxidation type.

In any of these laser diodes, it should be noted that the feature oflow-threshold current of laser oscillation is achieved by restrictingthe region of current injection to a particular area of the devicelocated at a central part of the device structure.

For example, the Non-Patent Reference 1 discloses a surface-emissionlaser diode having an active layer of InGaAs and oscillating at thewavelength band of 0.98 μm. In this surface-emission laser diode, thereis formed a selectively oxidized layer of Al_(0.98)Ga_(0.02)As in theupper Bragg reflector of the p-Al_(0.9)Ga_(0.1)As/GaAs structure formedabove the active layer.

Such a surface-emission laser diode is produced by the steps of: etchingthe upper distributed Bragg reflector, after the crystal growth processthereof, to form a square mesa structure in such a manner that thesidewall surface of the Al_(0.98)Ga_(0.02)As layer to be oxidized isexposed; and forming the selectively oxidized layer by applying aselective oxidizing process to the foregoing Al_(0.98)Ga_(0.02)As layerstarting from the mesa sidewall surface toward a mesa central region atthe temperature of 425° C. in a water vapor ambient produced by bubblingwater heated to 85° C. by a nitrogen gas.

As a result of the foregoing selective oxidation process, there isformed an insulation region of AlOx (oxide of Al) around the mesastructure, and associated with this, there is formed a conductive regionat the central part of the mesa structure in the form of a non-oxidizedregion.

Thus, the selectively oxidized region of a surface-emission laser diodeis generally formed by exposing a part of the AlGaAs selectivelyoxidizing layer to an ambient containing water vapor by an etchingprocess, or the like.

With the surface-emission laser diode of this type, the holes suppliedfrom the surface of the upper distributed Bragg reflector is injectedinto the active layer with confinement into the non-oxidized conductiveregion at the central part of the mesa structure. It should be notedthat AlOx is an excellent insulator and can effectively restrict theregion of hole injection to the active layer to the foregoing centralpart of the mesa structure. By using such a selectively oxidizedstructure, it becomes possible to reduce the oscillation thresholdcurrent drastically. In the Non-Patent Reference 2, a very low thresholdcurrent of 900 μA is achieved in the device having the non-oxidizedregion of 4.5 μm×8 μm.

Further, because of the fact that AlOx has a low refractive index ofabout 1.6, which is lower than the refractive index of othersemiconductor layers, the surface-emission laser diode of theselective-oxidation type has an advantageous feature in that the opticalbeam formed as a result of laser oscillation is confined at the centralpart of the mesa structure as a result of formation of the lateralrefractive index profile within the laser cavity structure. Thereby,optical loss caused by diffraction is reduced and the efficiency of thelaser diode is improved.

On the other hand, associated with the increased degree of opticalconfinement, it becomes necessary to decrease diameter of the oxidizedconfinement region with such a laser diode for suppressing highertransverse mode oscillation. While it depends on the wavelength band, itis known that a single fundamental mode oscillation can be achieved witha surface-emission laser diode having a single oxidized currentconfinement structure, by reducing the diameter or edge length of theoxidized confinement structure down to 3-4 times as large as theoscillation wavelength. Thus, by using such a selective oxidizingstructure, it becomes possible to achieve decrease of laser oscillationthreshold and decrease of diffraction loss, in addition to the singlefundamental mode control.

In the case of the surface-emission laser diode of the hydrogen ionimplantation type, on the other hand, no built-in waveguide structure isprovided contrary to the case of the surface-emission laser diode of theselective oxidation type.

In a surface-emission laser diode of the hydrogen ion implantation type,a waveguide structure is inducted by a refractive index change that iscaused at the time of operation of the laser diode by the heat flowingthrough the device (thermal lens effect), and confinement of thetransverse mode is achieved by using such a waveguide structure inducedby the thermal lens effect.

Because the optical confinement obtained with such a laser diode isgenerally weak, it is possible with the surface-emission laser diode ofthe hydrogen ion implantation type, to obtain a single fundamentaltransverse mode oscillation even in the case the diameter of currentconfinement is relatively large. In the Non-Patent Reference 3, there isdisclosed a surface-emission laser diode of the hydrogen ionimplantation type using GaAs for the active layer and operating at thewavelength band of 0.85 μm. With the laser diode of the Non-PatentReference 3, an oscillation threshold current of 2.5 mA is obtained byusing a hole injection region having a diameter of 10 μm.

In many applications of surface-emission laser diode, there exists arequest, in addition to the request for low threshold characteristics,in that the laser diode provides a single peak beam shape at high outputpower state. Thus, control of single transverse mode is a very importantobject in the surface-emission laser diode. Generally, insurface-emission laser diode, control of the single transverse mode ispossible only in the state in which the laser diode is operating at arelatively low current injection level. When the current injection levelis increased, there is caused oscillation of higher order transversemode due to the spatial hole-burning effect of carriers.

More specifically, when the laser diode is operating under the highcurrent injection state, there occurs an increase in the photon densityinside the optical cavity, while such an increase of the photon densityfacilitates increase of stimulated emission in the part where theoptical intensity is large. As a result of such an increased stimulatedemission, there is caused a localized dip of carrier density (spatialhole-burning phenomenon).

Because the fundamental transverse mode has a large mode amplitude(electric field distribution) at the central part of the mesa structure,there occurs a decrease of carrier density at such a central part of themesa structure with increase of the injection current, while thisdecrease of the carrier density at the mesa central part leads tosaturation of laser gain for the fundamental transverse mode. On theother hand, at the peripheral part of the mesa structure surrounding themesa central part, there occurs an increase of carrier density, and witthis, a laser oscillation is caused for the higher-order transverse modehaving a mode amplitude in the region between the mesa central part andthe mesa peripheral part because of increase of laser gain in such apart of the laser diode.

It should be noted that this phenomenon appears particularly conspicuousin the selective-oxidation type surface emission laser diode in whichthere is caused a strong mode confinement by using the selectiveoxidized structure. Thereby, the quality of the exiting laser beam isdeteriorated seriously.

In the case of the surface-emission laser diode of the hydrogen ionimplantation type, there is provided no such built-in opticalconfinement structure, and because of this, the laser diode shows poorstability for the transverse mode. Thus, the laser diode easily causeshigher mode oscillation when the injection current is increased.

In order to suppress the laser oscillation of higher order transversemode in such a surface-emission laser diode, various proposals have beenmade so far.

For example, the Non-Patent Reference 4 discloses an approach ofsuppressing the higher order transverse mode oscillation by forming anantiguiding structure in a part of the cavity structure, by selectivelyoxidizing a layer of Al_(0.9)Ga_(0.1)As from the crystal growth surfacein correspondence to the current injection region at the central part ofthe mesa structure.

In this example, there is formed an antiguiding structure, in asurface-emission laser diode oscillating at the 0.98 μm wavelength andhaving a current confinement structure of the diameter of 10-17 μm, thecurrent confinement structure being the one formed by selectiveoxidation of the AlAs selective oxidizing layer having the thickness of18.6 nm in a high-temperature water vapor ambient, starting from themesa etching sidewall surface, by selectively oxidizing a mixed crystalof Al_(0.9)Ga_(0.1)As having a thickness of ½λ, leaving the currentinjection region at the central part of the mesa structure over theregion of 15 μm in diameter. With this, the higher order transverse modeoscillation is effectively suppressed and a single peak radiationpattern is obtained even in the case a drive current twenty times aslarge as the threshold current is injected.

In the example of the Non-Patent Reference 4, the spatial overlapping ofthe higher order transverse mode and the gain region in the active layer(current injection region) is decreased by providing the antiguidingstructure at the central part of the cavity structure. With this, itbecomes possible to suppress the laser oscillation at the highertransverse mode.

FIGS. 3A and 3B are diagrams for explaining the effect of theantiguiding structure provided partially in the optical cavity on theelectric field distribution of the fundamental mode and the first-orderhigher mode. Here, it should be noted that FIG. 3A is a diagram showingthe mode distribution for the case the anti-guiding structure is notprovided, while FIG. 3B shows the mode distribution for the case anantiguiding structure is provided at the central part of the device. InFIGS. 3A and 3B, the upper diagram shows the fundamental transverse modedistribution, while the lower diagram shows the first-order transversemode distribution. Further, the gain region formed with the currentinjection is also represented in FIGS. 3A and 3B.

As can be seen from FIG. 3A, it is difficult to spread the modedistribution, in the case of the fundamental transverse mode having alarge mode distribution at the central part of the mesa structure,toward the peripheral direction of the mesa structure by the antiguidingstructure, while in the case of the higher order transverse mode, theelectric field strength is zero at the center of the mesa structure andthere appears a large mode distribution at the peripheral part of themesa structure as shown in FIG. 3B. Thus, it becomes possible with theantiguiding structure to deform and expand the mode distribution profilelaterally toward the mesa peripheral direction, and it becomes possibleto decrease the mode distribution (electric field strength) at the mesacentral part.

Thus, from the reason explained above, the surface-emission laser diodecan maintain the fundamental transverse mode operation up to high outputstate with such an antiguiding structure, which decreases the gain forthe higher order transverse modes. Further, with regard to thefundamental transverse mode, it is difficult to modify and expand themode distribution profile in the lateral direction by the antiguidingstructure, although it is also expected that the use of such anantiguiding structure results in somewhat broadening of the electricfield distribution at the mesa central part. Thereby, the electric fieldintensity at the mesa central part is decreased, and it is expected thatoccurrence of the spatial hole-burning phenomenon is suppressed.

Meanwhile, in the case of a surface-emission laser diode for use as theoptical source in the applications other than the optical communicationtechnology, there is a strong demand, in addition to the demand for thecircular beam shape suitable for optical coupling with optical fibers,in that the laser diode provides a single fundamental mode oscillation,while this requirement can be met, in the case of the surface-emissionlaser diode of the selective oxidation type, by setting the diameter ofthe current confinement structure to be less than about three times aslarge as the oscillation wavelength. In the case of the single-modedevice, on the other hand, increase of device resistance and electricalcapacitance associated with the selective oxidation structure imposes alimitation upon the modulation frequency band. Further, spatialhole-burning effect raises the problem that it is difficult to achievehigh-power laser oscillation in the single fundamental mode.

In the case of the surface-emission laser diode of the hydrogen ionimplantation type, on the other hand, there occurs no problem ofparasitic capacitance as in the case of the current confinementstructure formed by the selective oxidation process. In thesurface-emission laser diode of the hydrogen ion implantation type,there exists no built-in waveguide structure, and confinement oftransverse mode is achieved by the small refractive index change causedby heat generation, which in turn is caused primarily by the currentinjection. Thus, in the case of the surface-emission laser diode of thehydrogen ion implantation type, the degree of lateral confinement in thedevice is inherently weak, while this leads to the disadvantageousfeature of unstable transverse mode control associated with the weaklateral confinement, although there is obtained an advantageous featurethat it is possible to achieve single fundamental transverse modeoscillation for the case of relatively large confinement diameter.

Non-Patent Reference 5 discloses a surface-emission laser diode capableof maintaining the single fundamental transverse mode oscillation underthe state of laser oscillation with high output power, called ARROW(antiresonant reflecting optical waveguides) structure or S-ARROW(Simplified ARROW) structure.

In Non-Patent Reference 5, it should be noted that the object ofproviding a surface-emission laser diode structure capable ofmaintaining the single fundamental transverse mode oscillation under thestate of high laser oscillation power is attempted by way of using anantiguiding structure. Here, it should be noted that an antiguidingstructure is a waveguide structure in which there is provided a lowrefractive index core part in correspondence to the oscillation regionof the laser diode in such a manner that the low refractive index corepart is sandwiched by regions of relatively large refractive index inthe direction of the laser cavity. With such an antiguiding structure,leakage of higher-order transverse mode is facilitated in the directionperpendicular to the laser cavity direction, and with this, it becomespossible to maintain the single fundamental transverse mode oscillationup to the state of high output power.

Further, Non-Patent Reference 6 teaches an antiguiding structure inwhich there is provided a periodic structure formed of a low refractiveindex region and a high refractive index region adjacent to theoscillation region of the device constituting the low refractive indexcore such that there is formed a cavity structure in which the lowrefractive index core forms a half-wavelength resonator.

In this Non-Patent Reference 6, leakage of the transverse mode pertinentto the antiguiding structure is reduced, and with this, a singlefundamental transverse mode oscillation is achieved with the powerexceeding 7 mW.

Meanwhile, in the application of the surface-emission laser diode to thelong-range and super-fast optical communications, there arises aproblem, in addition to the single fundamental transverse modeoscillation, of noise caused by the change of reflectivity of theoptical components used therein depending on the polarization directionof the laser beam. Thus, there is a demand that the laser diode producesthe laser beam with polarization controlled to a specific polarizationdirection.

Conventionally, there has been disclosed a method of controlling thepolarization direction of laser beam by using a strained film asdisclosed in the Patent Reference 5, in which anisotropic stress isapplied to the laser diode or anisotropy is caused in the gain of theactive layer. Further, the Patent Reference 6 discloses a method offorming the surface-emission laser diode on an inclined substrate. Inany of these methods, there is induced an anisotropy in the gain causedin the active layer, and it becomes possible to control the polarizationdirection in the direction of large optical gain.

(Non-Patent Reference 1)

Applied Physics Letters vol. 66, No.25 pp.3413-3415, 1995 andElectronics Letters No.24, Vol.30, pp.2043-2044, 1994

(Non-Patent Reference 2)

Electronics, Letters No.24, Vol.30, pp.2043-2044, 1994

(Non-Patent Reference 3)

Electronics Letters No.5, Vol.27, 1991, pp.457-458

(Non-Patent Reference 4)

IEEE Photonics Technology Letters Vol. 10, No.1, 1998, pp.12-14

(Non-Patent Reference 5)

Applied Physics Letters vol.76, No.13, 2000, pp.1659 ; IEEE Journal ofQuantum Electronics vol.38, No.12, 2002, pp.1599.

(Non-Patent Reference 6)

IEEE Journal of Quantum Electronics, vol.38, No.12, pp.1599.

(Patent Reference 1)

Japanese Laid-Open Patent Application 11-54838

(Patent Reference 2)

Japanese Laid-Open Patent Application 20001-60739

In the surface-emission laser diode according to the Non-PatentReference 4 cited before, it should be noted that the antiguidingstructure of AlOx is provided inside the cavity. Thus, in order to formsuch an antiguiding structure, there is conducted a selective oxidationprocess of an AlGaAs layer having a large Al content in ahigh-temperature water vapor environment. This process will be referredto hereinafter as water vapor selective oxidation.

Now, in the case of forming such an antiguiding structure, it isnecessary to provide the AlOx layer constituting the antiguidingstructure at the central part of the cavity structure where thefundamental mode amplitude is large. This means that it is not possibleto apply a conventional method of oxidizing the selectively oxidizinglayer starting from the sidewall surface of the mesa structure as usedin the conventional surface-emission laser diode, and the selectiveoxidation has to be conducted from the device surface.

Further, in order to secure sufficient effect of the antiguidingstructure for the higher mode oscillation, it is necessary to providethe anti-guiding structure in correspondence to the part of the cavitywhere the electric field strength is relatively large. This means thatthe antiguiding structure has to be formed inside the upper distributedBragg reflector.

However, epitaxial growth of a semiconductor layer is not possible on anAlOx layer, and thus, it is necessary with such a surface-emission laserdiode to provide a dielectric distributed Bragg reflector on such anAlOx layer to form a laser structure. However, such a laser structurehas a drawback in that it requires not only an additional process ofevaporation deposition of the dielectric layers but also an etchingprocess for removing a part of the distributed Bragg reflector so as toenable injection of drive current via a top electrode.

Further, because there is a need of providing the selective oxidizinglayer used for forming the antiguiding structure always at the surfacepart of the device structure with the laser diode of such a structure inanticipation of the selective oxidation processing conducted in the hightemperature ambient containing water vapor, there is imposed arestriction on the degree of freedom of designing the device.

Further, because a dielectric multilayer mirror is provided in the upperpart of the laser diode with the surface-emission laser diode of thistype, the distance between the p-side electrode and the selectiveoxidation layer is reduced, and thus, the holes are injected into theconfinement region in the lateral direction from the peripheral part ofthe AlOx layer constituting the antiguiding structure. Thereby, thedevice resistance of the laser diode is inevitably increased.

Further, as noted before, it is necessary that the polarizationdirection of the output laser beam is controlled to a specific directionin the case the laser diode is to be used for the optical fibercommunications or for the optical source of electro-photographicsystems.

In the application of the surface-emission laser diode to high-speedoptical fiber communications, there arises a problem of noise caused bythe polarization-dependent difference of reflection or transmissioncharacteristics of the optical components constituting the system.Further, in the application of optical writing, too, there arises aproblem with the polarization dependence of the optical systems usedtherein that an optical beam spot is distorted on the surface of thephotosensitive body.

Because the foregoing reference does not describe the control ofpolarization direction, it is difficult to use the teaching thereindirectly for the laser diode device used for actual applications.

With regard to the attempt of controlling the polarization direction, itis proposed to form the laser diode on an inclined substrate or apply ananisotropic stress to the device as noted before. However, the formerapproach has a drawback in that the range of crystal growth conditionused at the time of the crystal growth process of the laser diodestructure is limited. Further, in the latter approach, controllabilityof polarization depends heavily on the controllability andreproducibility of processing condition.

Further, with the method of forming a surface-emission laser diode on aninclined substrate as described in the foregoing Patent Reference 2,there is a problem that adjustment and control of crystal growthcondition is difficult ion such an inclined substrate as compared withthe substrate having the (100) surface orientation. Further, while it ispossible to control the polarization direction of the laser oscillationbeam in a particular direction along a specific crystal orientation withsuch a method, it is difficult to control the polarization direction inan arbitrary direction for individual devices.

Further, with the method of applying an anisotropic strain to the activelayer by using the thin film accumulating therein a stress as set forthin Patent Reference 1, there arises a problem that the control ofpolarization becomes unstable because of the reproducibility andcontrollability of the processing condition.

SUMMARY OF THE INVENTION

The present invention provides a surface-emission laser diode capable ofincreasing the degree of freedom of design, capable of suppressinghigher order transverse mode oscillation more effectively, maintainingsingle fundamental transverse mode operation even under the operationalstate of high optical output, and capable of controlling thepolarization direction of the laser beam in a specific direction.Further, the present invention provides a surface-emission laser array,an optical interconnection system, an optical communication system, anelectro-photographic system and an optical disk system that uses such asurface-emission laser diode.

In a first aspect of the present invention, there is provided asurface-emission laser diode, comprising:

-   -   an active layer;    -   a pair of cavity spacer layers formed at both sides of said        active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across a structure formed of said active layer and said cavity        spacer layers, said current confinement structure being formed        by a selective oxidation process of a semiconductor layer,    -   said pair of distributed Bragg reflectors being formed of        semiconductor materials,    -   wherein there is provided a region containing an oxide of Al and        having a relatively low refractive index as compared with a        surrounding region in any of said semiconductor distributed        Bragg reflector or said cavity spacer layer in correspondence to        a part spatially overlapping with said current injection region        in a laser cavity direction.

In a second aspect of the present invention, there is provided asurface-emission laser diode, comprising:

-   -   an active layer;    -   a pair of cavity spacer layers formed at both sides of said        active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across a structure formed of said active layer and said pair of        cavity spacer layers,    -   said current confinement structure comprising a high resistance        region formed by an ion implantation process,    -   said pair of distributed Bragg reflectors being formed of        semiconductor materials,    -   wherein there is provided a region containing an oxide of Al and        having a relatively low refractive index than a surrounding        region in any of said semiconductor distributed Bragg reflector        or said cavity spacer layer in correspondence to a part        spatially overlapping with said current injection region in a        laser cavity direction.

In a third aspect of the present invention, there is provided asurface-emission laser diode, comprising:

-   -   an active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across said active layer;    -   said current confinement structure being formed of a selective        oxidation process of a semiconductor material,    -   said surface-emission laser diode comprising an AlGaAs mixed        crystal layer,    -   a part of said AlGaAs mixed crystal layer having spatial        overlapping with said current injection region in a laser cavity        direction,    -   said part of said AlGaAs mixed crystal being formed by selective        ion implantation of molecules containing oxygen and a thermal        annealing process following said selective ion implantation        process, said part of said AlGaAs mixed crystal having a        relatively low refractive index as compared with a surrounding        region wherein ion implantation of molecules containing oxygen        is not made and located on an identical plane of said part, said        plane being perpendicular to said laser cavity direction.

In a fourth aspect of the present invention, there is provided asurface-emission laser diode, comprising:

-   -   an active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across said active layer,    -   said current confinement structure comprising a high resistance        region formed by an ion implantation process,    -   said surface-emission laser diode comprising an AlGaAs mixed        crystal layer,    -   a part of said AlGaAs mixed crystal layer having a spatial        overlapping with said current injection region in a laser cavity        direction,    -   said part of said AlGaAs mixed crystal spatially overlapping        with said current injection region being processed by an ion        implantation process of molecules containing oxygen and a        thermal annealing process following said ion implantation        process and having a relatively low refractive index as compared        with a surrounding region not injected with molecules containing        oxygen and located on an identical plane of said part, said        plane being perpendicular to said laser cavity direction.

In a fifth aspect of the present invention, there is provided asurface-emission laser diode according to any of the first throughfourth aspects, wherein there is provided any of a GaAs layer or a GaInPmixed crystal layer adjacent with said AlGaAs mixed crystal layerconstituting said region of relatively low refractive index with respectto said surrounding region not injected with said molecules containingoxygen and located on said identical plane of said part where said ionimplantation of said molecules has been made.

In a sixth aspect of the present invention, there is provided a surfaceemission laser diode according to any of the first through fifthaspects, wherein said AlGaAs mixed crystal layer, constituting saidregion of relatively low refractive index with respect to saidsurrounding region not made with ion implantation of moleculescontaining oxygen and located on said identical plane perpendicular tosaid laser cavity direction, is provided at a location corresponding toan anti-node of a standing wave of laser oscillation occurring in saidcavity structure, said AlGaAs mixed crystal layer being doped to ahigher concentration level as compared with other AlGaAs mixed crystallayers constituting said surface-emission laser diode.

In a seventh aspect of the present invention, there is provided asurface-emission laser diode according to any of said first throughsixth aspects, wherein said region of relatively low refractive index isprovided in plural numbers.

In an eighth aspect of the present invention, there is provided asurface-emission laser diode according to any of the first throughseventh aspects, wherein said region of relatively low refractive indexis provided inside an n-type distributed Bragg reflector constitutingone of said pair of distributed Bragg reflectors.

In a ninth aspect of the present invention, there is provided asurface-emission laser diode according to any of the first througheighth aspects, wherein there is further provided a region of relativelyhigh refractive index around said region of relatively low refractiveindex provided in spatial overlapping with said laser cavity region insaid laser cavity direction, and wherein there is further provided acladding region of low refractive index with respect to said highrefractive region such that said cladding region is located around saidregion of high refractive index.

In a tenth aspect of the present invention, there is provided asurface-emission laser diode according to the ninth aspect, whereinthere is provided an anisotropy in a width of said high refractiveregion surrounded by said cladding region.

In an eleventh aspect of the present invention, there is provided asurface-emission laser diode according to the ninth aspect, wherein saidcladding region is provided in a pair in a direction perpendicular tosaid laser cavity direction across a laser cavity region.

In a twelfth aspect of the present invention, there is provided asurface-emission laser diode according to any of the first througheleventh aspects, wherein said region of relatively low refractive indexhas an anisotropic shape.

In a thirteenth aspect of the present invention, there is provided asurface-emission laser diode according to any of the first throughtwelfth aspects, wherein said active layer comprises a group III-Vcompound semiconductor material, said group III element comprises atleast one of Ga and In, and wherein said group V element comprises oneor more of As, N, Sb and P.

In a fourteenth aspect of the present invention, there is provided asurface-emission laser array comprising a plurality of surface-emissionlaser diodes each according to any of first through thirteenth aspects,said plurality of surface-emission laser diodes being arranged to forman array.

In a fifteenth aspect of the present invention, there is provided anoptical interconnection system having an optical source comprising asurface-emission laser diode according to any of the first throughthirteenth aspect of the present invention or an surface-emission laserarray according to a fourteenth aspect of the present invention.

In a sixteenth aspect of the present invention, there is provided anoptical communication system that uses the surface-emission laser diodeaccording to any of the first through thirteenth aspects of the presentinvention or the surface-emission laser array according to thefourteenth aspect of the present invention.

In a seventeenth aspect of the present invention, there is provided anelectro-photographic system using any of the surface-emission laserdiode according to any of the first through thirteenth aspect of thepresent invention or the surface-emission laser array according to thefourteenth aspect of the present invention.

In an eighteenth aspect of the present invention, there is provided anoptical disk system that uses the surface-emission laser diode accordingto any of the first through thirteenth aspect of the present inventionor the surface-emission laser array according to the fourteenth aspectof the present invention as the reading/writing optical source.

Another object of the preset invention is to provide a surface-emissionlaser diode capable of controlling the polarization direction in anarbitrarily determined desired direction while producing high out putpower in single fundamental transverse mode laser oscillation andfurther capable of conducting high speed modulation.

In a nineteenth aspect of the present invention, there is provided asurface-emission laser diode constructed on a substrate having asubstrate surface, comprising:

-   -   an active layer parallel to said substrate surface;    -   a pair of cavity spacer layers provided so as to sandwich said        active layer;    -   a pair of distributed Bragg reflectors provided across said        active layer and said cavity spacer layers so as to sandwich        said active layer and said cavity spacer layers therebetween;    -   a high resistance region defining a current injection region for        injecting a current into said active layer,    -   laser oscillation being caused in a laser cavity region between        said pair of distributed Bragg reflectors acting as cavity        mirrors in a direction perpendicular to said substrate surface        in correspondence to said current injection region,    -   a refractive index structure provided in a plane parallel to        said substrate surface, said refractive index structure        comprising: a low refractive index core including said laser        cavity region at a central part thereof; and a periodic        structure provided around said low refractive index core, said        periodic structure comprising a low refractive region        surrounding said low refractive index core and a high refractive        index region surrounding said low refractive index region, said        low refractive index region and said high refractive index        region being repeated alternately in said plane parallel to said        substrate,    -   wherein any of a width of said low refractive index core or a        shape of said periodic structure is changed between a specific        direction parallel to said substrate surface and other        directions parallel to said substrate surface different from        said specific direction.

In a twentieth aspect of the present invention, there is provided asurface-emission laser diode according to the nineteenth aspect of thepresent invention, wherein said low refractive index core has a widthdifferent between two directions parallel to said substrate surface andcrossing perpendicularly with each other.

In a twenty first aspect of the present invention, there is provided asurface-emission laser diode according to the nineteenth aspect of thepresent invention, wherein said periodic structure is different betweentwo directions parallel to said substrate surface and crossingperpendicularly with each other.

In a twenty second aspects of the present invention, there is provided asurface-emission layer diode according to any of the twentieth or twentyfirst aspects of the present invention, wherein a reflection wavelengthband of said periodic structure is set, in one of said two directionscrossing perpendicularly with each other, to be longer than a wavelengthof a fundamental transverse mode in said same direction and projectedupon said substrate surface.

In a twenty third aspects of the present invention, there is provided asurface-emission laser diode constructed on a substrate having asubstrate surface, comprising:

-   -   an active layer parallel to said substrate surface;    -   a pair of cavity spacer layers provided so as to sandwich said        active layer;    -   a pair of distributed Bragg reflectors provided across said        active layer and said cavity spacer layers so as to sandwich        said active layer and said cavity spacer layers therebetween;    -   a high resistance region defining a current injection region for        injecting a current into said active layer,    -   laser oscillation being caused in a laser cavity region between        said pair of distributed Bragg reflectors acting as cavity        mirrors in a direction perpendicular to said substrate surface        in correspondence to said current injection region, a refractive        index structure provided in a plane parallel to said substrate        surface, said refractive index structure comprising: a low        refractive index core including said laser cavity region at a        central part thereof; and a periodic structure provided around        said low refractive index core, said periodic structure        comprising a low refractive region surrounding said low        refractive index core and a high refractive index region        surrounding said low refractive index region, said low        refractive index region and said high refractive index region        being repeated alternately in said plane parallel to said        substrate,    -   wherein said periodic structure is provided partially in said        plane parallel to said substrate surface in a specific        direction.

In a twenty fourth aspect of the present invention, there is provided asurface-emission laser diode according to any of the nineteenth throughtwenty third aspects of the present invention, wherein said active layeris formed of a III-V semiconductor material, said active layercontaining any or both of Ga and In as a group III element, said activelayer containing any or all of As, N, Sb and P as a group V element.

In a twenty fifth aspect of the present invention, there is provided asurface-emission laser array formed of a surface-emission laser diodeaccording to any of the nineteenth through twenty fourth aspects of thepresent invention.

In a twenty sixth aspect of the present invention, there is provided anoptical interconnection system that uses the surface-emission laserdiode according to any of the nineteenth through twenty fourth aspectsof the present invention or the surface-emission laser array accordingto the twenty fifth aspect of the present invention.

In a twenty seventh aspect of the present invention, there is providedan optical communication system that uses the surface-emission laserdiode according to any of the nineteenth through twenty fourth aspectsof the present invention or the surface-emission laser array accordingto the twenty fifth aspect of the present invention.

In a twenty eighth aspect of the present invention, there is provided anelectrophotographic system that uses the surface-emission laser diode ofany of the nineteenth through twenty fourth aspects of the presentinvention or the surface-emission laser array according to the twentyfifth aspect of the present invention.

In a twenty ninth aspect of the present invention, there is provided anoptical disk system that uses a surface emission laser diode accordingto any of the nineteenth through twenty fourth aspects of the presentinvention or the surface-emission layer array according to the twentyninth aspect of the present invention.

According to the first aspect of the present invention, in which thereis provided a surface-emission laser diode, comprising: an active layer;a pair of cavity spacer layers formed at both sides of said activelayer; a current confinement structure defining a current injectionregion into said active layer; and a pair of distributed Braggreflectors opposing with each other across a structure formed of saidactive layer and said cavity spacer layers, said current confinementstructure being formed by a selective oxidation process of asemiconductor layer, said pair of distributed Bragg reflectors beingformed of semiconductor materials, wherein there is provided a regioncontaining an oxide of Al and having a relatively low refractive indexas compared with a surrounding region in any of said semiconductordistributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction, it becomes possible toprovide a selectively-oxidized surface-emission laser diode having anantiguiding structure characterized by low manufacturing cost, highdegree of freedom of design and low device resistance.

Thus, according to the first aspect of the present invention, the degreeof freedom of design is increased in the selectively oxidizedsurface-emission laser diode having the anti-guiding structure formed ofan oxide of Al, and it becomes possible to provide a device in whichoscillation of higher transverse mode is suppressed more effectively andis capable of providing high output operation in the single fundamentaltransverse mode

According to the second aspect of the present invention, in which thereis provided a surface-emission laser diode, comprising: an active layer;a pair of cavity spacer layers formed at both sides of said activelayer; a current confinement structure defining a current injectionregion into said active layer; and a pair of distributed Braggreflectors opposing with each other across a structure formed of saidactive layer and said pair of cavity spacer layers, said currentconfinement structure comprising a high resistance region formed by anion implantation process, said pair of distributed Bragg reflectorsbeing formed of semiconductor materials, wherein there is provided aregion containing an oxide of Al and having a relatively low refractiveindex than a surrounding region in any of said semiconductor distributedBragg reflector or said cavity spacer layer in correspondence to a partspatially overlapping with said current injection region in a lasercavity direction it becomes possible to obtain a hydrogen ionimplantation type surface-emission laser diode having an antiguidingstructure of an Al oxide in which the degree of freedom of design isincreased and device resistance is decreased.

Thus, according to the second aspect of the present invention, thedegree of freedom of design is increased in the hydrogen-ionimplantation type surface-emission laser diode having the anti-guidingstructure formed of an oxide of Al, and it becomes possible to provide adevice in which oscillation of higher transverse mode is suppressed moreeffectively and is capable of providing high output operation in thesingle fundamental transverse mode

Further, according to the third aspect of the present invention, thedegree of freedom of design is increased and the oscillation of highertransverse mode is suppressed more effectively, and it becomes possibleto provide a surface-emission laser diode capable of providing highoutput operation in the single fundamental transverse mode.

In the prior art device according to Non-Patent Reference 4, it shouldbe noted that there is provided a low refractive index region containingan oxide of Al as an antiguiding structure in the vicinity of the laseroscillation region. In order to obtain such a structure, the referenceuses the process of selectively oxidizing, after the step of forming thecavity spacer layer and a part of the distributed Bragg reflector, thesemiconductor layer containing Al at the surface in an ambientcontaining water vapor, such that there is formed a low refractive indexregion of oxide of Al in correspondence to the part spatiallyoverlapping with the laser oscillation region.

Thus, there is formed a region of oxide on the surface of the partlyformed device structure, it is difficult to continue crystal growth ofsemiconductor material with such a conventional process, and because ofthis, the prior art device has a construction that uses a dielectricdistributed Bragg reflector for the upper reflector.

However, the use of such a distributed Bragg reflector imposes aconstraint on the current injection path and there arises a tendencythat the device resistance is increased as a result of such aconstraint. Further, such a process requires an evaporation depositionapparatus for dielectric films, in addition to the growth apparatus ofthe semiconductor films. Thereby, it becomes necessary to adjust thedeposition rate equal in both deposition apparatuses in order to achieveprecise film thickness control at the time of formation of thedistributed Bragg reflector. Thereby, the cost for production of thedevice is increased.

Such a problem of conventional art can be resolved by forming an oxideregion inside the semiconductor structure without causing oxidation atthe outermost surface of the device structure. Such formation of theoxide region can be achieved for example by implantation of oxygen ionsfrom the device surface by an ion implantation process. After theimplantation of the oxygen ions, a thermal annealing process isconducted, and with this, only the part of the device structure wherethe oxygen ions have been injected is selectively oxidized.

Thus, according to the third aspect of the present invention, it becomespossible to form the low refractive index region necessary for formationof the antiguiding structure by conducting ion implantation of oxygenions from the crystal growth surface, followed by a thermal annealingprocess. Thereby, the low refractive index layer is formed selectivelyin correspondence to the region where the implantation of the oxygenions has been made.

It should be noted that Japanese Laid-Open Patent Application 9-27650and Japanese Laid-Open Patent Application 2002-289967 describe thatoxidation of an AlGaAs mixed crystal is possible by an ion implantationprocess of oxygen ions or molecules containing oxygen, followed by athermal annealing process. Thereby, it should be noted that an AlGaAsmixed crystal is oxidized with high rate when the Al content thereof islarge. On the other hand, there occurs little oxidation in a GaAs layer.

Thus, the foregoing prior art references merely discloses the technologyof building a waveguide structure inside an optical cavity by oxidizingthe AlGaAs region outside the cavity region upon completion of thecrystal growth process. Associated therewith, the prior art referencesachieves oxidation of the entire epitaxial layers from the devicesurface down to the substrate. There is no reference in these prior artreferences about formation of the antiguiding structure or regrowth ofsemiconductor layers resumed after the crystal growth process.

With the foregoing oxidation process of AlGaAs layer by implantation ofoxygen ions, it is possible to selectively cause oxidation in a specificAlGaAs layer in an epitaxial structure without exposing the layer to beoxidized to the high temperature ambient containing water vapor,contrary to the case of the conventional device, by choosing the ionimplantation depth and ion implantation area. Thus, there is no need ofexposing the AlGaAs layer at the device surface and apply an oxidationprocess. Further, there is formed no AlOx layer at the device surface.Thus, it is possible to resume the crystal growth of the device part bymerely recovering the defects caused by the ion implantation process byconducting a thermal annealing process. Thereby, it becomes possible toform an antiguiding structure in a desired region inside the devicestructure easily, and the degree of freedom of design of the laser diodeis improved significantly.

Because it is merely required that the oxidation structure constitutingthe antiguiding structure at the mesa central part has a relatively lowrefractive index as compared with the surrounding part, there is no needthat the AlGaAs mixed crystal is oxidized completely. In other words,with the present invention, it is possible to adjust the dose of theoxygen ions such that the desired refractive change is achieved. Thus,with the present invention, the degree of freedom of design of thesurface-emission laser diode having the antiguiding structure of lowrefractive index is increased significantly.

Thus, according to the third aspect of the present invention, it becomespossible to form the antiguiding structure, in a surface-emission laserdiode having a current confinement structure formed by a selectiveoxidation process, at a desired region inside the device structure byconducting the selective oxidation of the AlGaAs mixed crystal in theform of an ion implantation process of molecules containing oxygen,followed by a thermal annealing process. Thereby, the degree of freedomof design of the device is increased, and it becomes possible to obtaina device operable at high output power state in the single fundamentaltransverse mode while suppressing the higher order transverse modeoscillation more efficiently.

According to the fourth aspect of the present invention, in which thereis provided a surface-emission laser diode, comprising: an active layer;a current confinement structure defining a current injection region intosaid active layer; and a pair of distributed Bragg reflectors opposingwith each other across said active layer, said current confinementstructure comprising a high resistance region formed by an ionimplantation process, said surface-emission laser diode comprising anAlGaAs mixed crystal layer, a part of said AlGaAs mixed crystal layerhaving a spatial overlapping with said current injection region in alaser cavity direction, said part of said AlGaAs mixed crystal spatiallyoverlapping with said current injection region being processed by an ionimplantation process of molecules containing oxygen and a thermalannealing process following said ion implantation process and having arelatively low refractive index as compared with a surrounding regionnot injected with molecules containing oxygen and located on anidentical plane of said part, said plane being perpendicular to saidlaser cavity direction, the degree of freedom of design of the device isincreased, and a surface-emission laser diode capable of providing ahigh output power operation in the single fundamental transverse mode isobtained, while suppressing higher-order transverse mode oscillationmore effectively.

Thus, with the hydrogen ion implantation type surface-emission laserdiode of the fourth aspect of the present invention that achieves thecurrent confinement by forming a high resistance region by ionimplantation of hydrogen ions, it is also possible to form anantiguiding structure at an arbitrary region inside the device structuresimilarly to the case of the device of the third aspect of the presentinvention, and the degree of freedom of design of the laser diode isincreased.

Because a hydrogen-ion implantation type surface-emission laser diodelacks a built-in waveguide structure, it is possible with such a laserdiode to change the mode distribution easily by causing a small changeof the refractive index. Thus, in the case such an antiguiding structureis formed in the optical cavity, it becomes possible with thesurface-emission laser diode of the fourth aspect of the presentinvention to move the part of the higher-order transverse mode where themode amplitude is large-to the region outside the current injectionregion (gain region). Thereby, it becomes possible to suppress the laseroscillation at the higher transverse mode more effectively.

Because lateral optical confinement is weak in a surface-emission laserdiode of hydrogen ion implantation type, the laser diode of this typehas an advantageous feature of causing a single fundamental transversemode oscillation even when the current confinement diameter is large. Onthe other hand, the laser diode has a disadvantageous feature, becauseof the lack of the built-in waveguide structure, that the transversemode is unstable and there occurs higher mode oscillation easily whenthe level of drive current injection is increased.

By forming a slight antiguiding structure partially as set forth in thesurface-emission laser diode of the fourth aspect of the presentinvention, it is now easy to move the part of the higher transverse modewhere the mode amplitude is large away from the gain region.

Further, because the laser diode of the fourth aspect of the presentinvention is the hydrogen ion implantation type surface emission laserdiode, it becomes possible with the fourth aspect of the presentinvention to increase the current confinement diameter as compared withthe surface-emission laser diode of the oxide-confinement type. Thus,the laser diode of the fourth embodiment has and advantageous feature ofrealizing high output operation easily.

In view of high susceptibility of mode distribution on the variation ofthe refractive index in such a hydrogen ion implantation typesurface-emission laser diode that lacks the built-in waveguidestructure, it is preferable that the antiguiding structure is providedin the laser diode according to the fourth aspect of the presentinvention at a location away from the active region where there iscaused large optical intensity for reducing the effect on thefundamental transverse mode. Further, in the case the antiguidingstructure is provided by way of oxygen ion implantation and thermalannealing processing, it is possible to suppress the magnitude ofrefractive index change by adjusting the amount of the injected oxygenions.

Thus, with the hydrogen ion implantation type surface-emission laserdiode of the fourth aspect of the present invention, the laseroscillation at higher-order transverse mode is suppressed effectivelyand high output power is obtained in the single fundamental transversemode oscillation.

According to the fifth aspect of the present invention, in which thereis provided a surface-emission laser diode configured according to anyof the first through fourth aspects of the present invention, whereinthere is provided any of a GaAs layer or a GaInP mixed crystal layeradjacent with said AlGaAs mixed crystal layer constituting said regionof relatively low refractive index with respect to said surroundingregion not injected with said molecules containing oxygen and located onsaid identical plane of said part where said ion implantation of saidmolecules has been made, it becomes possible to limit the oxidizedregion, and it becomes possible to adjust the phase condition moreprecisely.

With the technology of ion implantation, injected ions generallydistribute with a continuous depth profile having a peak correspondingto the acceleration voltage used for the ion implantation process.

For example, when the acceleration voltage is low and the peak depth ofion implantation is shallow, the injected ions distribute with a sharpdepth profile. Thereby, it is easy to oxidize an AlGaAs mixed crystallayer of desired depth selectively. When the ion implantation is to beconducted to a depth larger than a certain depth, the depth profile ofthe injected ions is broadened, and oxidation is induced also in theAlGaAs layers adjacent to the AlGaAs layer to be oxidized. Thereby,there is caused a problem that the refractive index is changedunwontedly. In a distributed Bragg reflector, it is preferable that theAlGaAs layer in the antiguiding structure has a thickness satisfying thephase condition of multiple reflection of a Bragg reflector after it isoxidized, while such unpredicted change of refractive index causes adecrease in the reflectivity of the Bragg reflector.

On the other hand, by providing a GaInP mixed crystal layer or a GaAslayer or a GaInP/GaAs distributed Bragg reflector (semiconductorstructure) adjacent to the Bragg reflector including the AlGaAs layer tobe selectively oxidized, the extent of the oxide region is restricted,and it becomes possible to adjust the phase condition more precisely.Here, it should be noted that GaInP can be grown continuously to a GaAssubstrate with lattice matching therewith. Because GaInP does notcontain Al as a constituent element, there is caused no substantialprogress of oxidation even when the oxygen ion implantation has beenmade. Thereby, it becomes possible to oxidize a specific AlGaAs layerselectively even in the case there is a depth distribution in theprofile of the injected ions. By selectively oxidizing a specific AlGaAslayer of a desired position, it becomes possible to control thereflection wavelength (reflection wavelength to the fundamentaltransverse mode) of the distributed Bragg reflector at the mesa centralpart, and a laser diode having excellent device characteristics such asoscillation threshold characteristics is obtained.

According to the sixth aspect of the present invention, in which thereis provided a surface emission laser diode configured according to anyof the first through fifth aspects of the present invention, whereinsaid AlGaAs mixed crystal layer, constituting said region of relativelylow refractive index with respect to said surrounding region not madewith ion implantation of molecules containing oxygen and located on saididentical plane perpendicular to said laser cavity direction, isprovided at a location corresponding to an anti-node of a standing waveof laser oscillation occurring in said cavity structure, said AlGaAsmixed crystal layer being doped to a higher concentration level ascompared with other AlGaAs mixed crystal layers constituting saidsurface-emission laser diode, occurrence of higher transverse modeoscillation is suppressed more effectively, and a device capable ofoperating at high output power in the single fundamental transverse modeis obtained.

It should be noted that a semiconductor material doped to a highconcentration level shows conspicuous optical absorption by themechanism of free carrier absorption, and the like. In addition to this,a p-type semiconductor material has a property that the opticalabsorption of long wavelength band is large because of the intra-valenceband absorption. Thus, it is possible to increase the free carrierabsorption or the absorption loss by intra-valence band absorption inthe AlGaAs layer that forms the low refractive index layer uponapplication of the selective oxidation processing by the oxygen ionimplantation and subsequent thermal annealing process, when the AlGaAslayer is doped to high concentration level at the time of crystal growththereof as compared with the surrounding AlGaAs layers. Thereby, itshould be noted that, because of the formation of the insulatingmaterial of AlOx at the part where the oxygen ion implantation has beenmade after the foregoing thermal annealing process, such a partundergoes a change to a transparent state with the formation of the AlOxinsulating material. Thereby, the absorption loss for the fundamentaltransverse mode, which has a large mode amplitude at the mesa centralpart (ion injected region), is vanished in correspondence to the mesacentral part, while there is caused a strong absorption for thehigher-order transverse mode having a large mode amplitude in the regionbetween the central part and the peripheral part of the mesa structure.Thereby, it becomes possible to effectively suppress the laseroscillation with the higher-order transverse mode.

In a distributed Bragg reflector, there is formed a standing wave withsuch a distribution that a node and an anti-node are repeatedalternately in each optical thickness of ¼λ. Thus, in the case of anne-cavity, the interface transiting from the AlGaAs layer (lowrefractive index layer) to the GaAs layer (high refractive index layer)as viewed from the cavity corresponds to the node of the electric fieldwhile the interface transiting from the GaAs layer to the AlGaAs layercorresponds to the anti-node of the electric field.

By providing a highly doped AlGaAs layer in correspondence to theanti-node of the standing wave and by selectively oxidizing the part ofthe AlGaAs layer corresponding to the center of the oscillation regionby the oxygen ion implantation process and the thermal annealingprocess, absorption of the higher-order transverse mode, which has alarge electric field strength at the part of the anti-node, is increasedand it becomes possible to obtain a larger effect of suppressing thehigher-order transverse mode.

According to the seventh aspect of the present invention, in which thereis provided a surface-emission laser diode configured according to anyof said first through sixth aspects of the present invention, whereinsaid region of relatively low refractive index is provided in pluralnumbers, it becomes possible to obtain a surface-emission laser diode inwhich the degree of freedom of design is improved and is capable ofoperating more efficiently at high output power while maintaining thesingle fundamental transverse mode.

Thus, with the seventh aspect of the present invention, the adjustmentof antiguiding function is achieved easily by providing pluralantiguiding structures of a region of relatively low refractive indexwith the surrounding regions, and the degree of freedom of design isincreased. Further, by providing plural antiguiding structures, a largeantiguiding function is attained, and the higher-order transverse modeis suppressed more effectively.

Further, according to the eight aspect of the present invention, inwhich there is provided a surface-emission laser diode according to anyof the first through seventh aspects, wherein said region of relativelylow refractive index is provided inside an n-type distributed Braggreflector constituting one of said pair of distributed Bragg reflectors,it becomes possible to obtain a device having a reduced deviceresistance and capable of operating with high output power whilemaintaining a single fundamental transverse mode operation.

It should be noted that the carrier mobility is one order smaller in ap-type semiconductor material as compared with an n-type semiconductormaterial. Thus, a p-type semiconductor material inherently shows a highresistance. Because of this, the holes circumvent the oxidation regionconstituting the antiguiding structure, in the case of the device of theNon-Patent Reference 4 in which the oxidation region is provided in ap-type semiconductor layer so as to overlap with the conductive regionformed by the current confinement structure and are then injected to thecentral part of the mesa structure as a result of the currentconfinement action of the current confinement structure. Thus, therearises a drawback of large and limited current path. This problem ofincrease of resistance is enhanced because of the high resistance of thep-type semiconductor material.

By providing the antiguiding structure in the n-type distributed Braggreflector as in the eighth aspect of the present invention, on the otherhand, increase of resistance is held minimum even in the case thereexists an insulation region or high resistance region at the centralpart of the mesa structure in view of the fact that there is provided nocurrent confinement structure between the antiguiding structure and theactive layer and in view of the inherently large electron mobility.Thus, it becomes possible to suppress the device resistance generallyequal to that of the conventional device.

With the device of the present invention, it is possible to form theantiguiding structure inside the epitaxial structure by conducting theoxygen ion implantation and thermal annealing process. Thereby, there iscaused no formation of AlOx at the surface part, contrary to the devicethat has been applied with the selective oxidation process in a hightemperature ambient containing water vapor starting from the surfacepart, while this allows regrowth of semiconductor crystal layers, andthe semiconductor structure for the surface-emission laser diode isobtained easily. Further, it should be noted that the function of theantiguiding structure is generally the same with regard to thetransverse mode when it is provided in the p-type distributed Braggreflector or when it is provided in the n-type distributed Braggreflector. Thus, with the present invention, it is also possible toachieve the effect of suppressing the higher transverse mode oscillationsimilarly to the other surface-emission laser diodes. Further, as aresult of reduced device resistance, heating of the device is suppressedwith the laser diode, and the laser diode according to the eight aspectof the present invention can be operated with high output power.

Thus, as noted above, it becomes possible to obtain a device capable ofperforming a high power operation in the single fundamental transversemode.

According to the ninth aspect of the present invention, in which thereis provided a surface-emission laser diode configured according to anyof the first through eighth aspects of the present invention, whereinthere is further provided a region of relatively high refractive indexaround said region of relatively low refractive index provided inspatial overlapping with said laser cavity region in said laser cavitydirection, and wherein there is further provided a cladding region oflow refractive index with respect to said high refractive region suchthat said cladding region is located around said region of highrefractive index, the leakage (diffraction) loss of light is prevented,and single fundamental transverse mode oscillation becomes possible upto higher output power.

Thus, according to the ninth aspect of the present invention, it becomespossible to provide a surface-emission laser diode having an antiguidingstructure of oxide of Al, in which laser oscillation in highertransverse mode is suppressed more efficiently and is capable performinghigh output operation in the single fundamental transverse mode.

According to the tenth aspect of the present invention, in which thereis provided a surface-emission laser diode configured according to theninth aspect of the present invention, wherein there is provided ananisotropy in a width of said high refractive region surrounded by saidcladding region, the polarization direction of the laser beam iscontrolled in a specific direction, and that the laser diode canmaintain the single fundamental transverse mode operation up to furtherhigher output power.

Thus, according to the present invention, higher transverse mode laseroscillation is suppressed further effectively with the surface-emissionlaser diode having the antiguiding structure of oxide of Al, byproviding the cladding region having an anisotropic shape. Thereby, alaser beam having a polarization direction in a specific direction isobtained.

According to the eleventh aspect of the present invention, in whichthere is provided a surface-emission laser diode configured according tothe ninth aspect of the present invention, wherein said cladding regionis provided in a pair in a direction perpendicular to said laser cavitydirection across a laser cavity region, in other words, a pair ofcladding regions of low refractive index formed of oxide of Al outsidethe region including the laser oscillation region at the central partthereof are provided only in one direction across the cavity region, thepolarization direction is controlled in a specific direction and singlefundamental mode operation becomes possible up to further higher outputpower.

Thus, according to the eleventh aspect of the present invention, laseroscillation in the higher-order transverse mode is suppressed furthereffectively with the surface-emission laser diode having an antiguidingstructure of an oxide of Al by providing a cladding region having ananisotropic shape, wherein the laser diode of such a construction canproduce a laser beam with polarization direction thereof controlled in aspecific direction and can provide with high output power whilemaintaining the single fundamental transverse mode operation.

According to the twelfth aspect of the present invention, in which thereis provided a surface-emission laser diode according to any of the firstthrough eleventh aspects, wherein said region of relatively lowrefractive index has an anisotropic shape, in other words, the region ofrelatively low refractive index, formed of oxide of Al and provided inspatial overlapping with the laser oscillation region for formation ofthe antiguiding structure, is formed to have an anisotropic form such asrectangular form or elliptical form, the polarization plane of the laserbeam is controlled in a specific direction. Further, the laser diode canmaintain single fundamental transverse mode operation up to high outputpower state.

Thus, according to the twelfth aspect of the present invention, thehigher-order transverse mode oscillation is suppressed furthereffectively in a surface-emission laser diode having an antiguidingstructure of oxide of Al, by providing the low refractive index regionconstituting the anti-guiding structure to have an anisotropic shape.Thereby, the laser oscillation at higher transverse mode is suppressedfurther effectively, and a laser beam having a polarization planecontrolled in a specific direction is obtained.

According to the thirteenth aspect of the present invention, in whichthere is provided a surface-emission laser diode according to any of thefirst through twelfth aspects, wherein said active layer comprises agroup III-V compound semiconductor material, said group III elementcomprises at least one of Ga and In, and wherein said group V elementcomprises one or more of As, N, Sb and P, it becomes possible to obtaina surface-emission laser diode having an oscillation wavelength in therange of 1.1 μm to 1.6 μm on a GaAs substrate, by using the foregoingmaterial for the active layer.

On a GaAs substrate, it is possible to provide a distributed Braggreflector that uses an AlGaAs mixed crystal having excellentcharacteristics, and with this, it becomes possible to obtain a laserdiode of excellent characteristics. Among these materials, GaInNAs inwhich nitrogen is added to GaInAs with an amount of several percent orless is particularly suitable for the active layer of the laser diode inview of it large conduction band discontinuity with regard to thebarrier layer of GaAs, and the like. Because of this, the laser diode ofthe thirteenth aspect of the present invention has a superiortemperature characteristic to the conventional laser diode of the samewavelength band and formed on an InP substrate.

Further, according to the first through twelfth aspects of the presentinvention, the laser diode maintains the single transverse modeoscillation up to high output power state, and a large couplingefficiency is realized with regard to an optical fibber, or the like.Thus, a surface-emission laser diode suitable for optical fibercommunication is obtained.

Further, according to the fourteenth aspect of the present invention, inwhich the surface-emission laser diode of any of the first throughthirteenth aspect of the present invention are arranged to form anarray, it becomes possible to provide a surface-emission laser arraycapable of maintain the single fundamental mode laser oscillation up tohigh output power stated.

Thus, according to the fourteenth aspect of the present invention, itbecomes possible to obtain a surface-emission laser array capable ofmaintaining the fundamental transverse mode laser oscillation up to thehigh output state and producing high quality laser beams. Thus, thesurface emission laser array of the present invention is suitable forthe optical source of multiple beam writing system for use in anelectro-photographic system or for the optical source of long-rangeoptical communication system.

According to the fifteenth aspect of the present invention, in whichthere is provided an optical interconnection system that uses any of thesurface-emission laser diode of any of the first through thirteenthaspect of the present invention or the surface-emission laser array ofthe fourteenth aspect of the present invention as the optical source, itbecomes possible to provide an optical interconnection system of highreliability.

Thus, with the surface-emission laser diode of the surface-emissionlaser array of the present invention, the fundamental transverse modelaser oscillation is maintained up to high output power state, and withthis, large optical coupling is achieved with regard to an opticalfiber. Further, because the laser oscillation of higher-order transversemode is effectively suppressed, there occurs little change of opticalinjection into the optical fiber even in the case the output state ofthe device is changed and hence the optical coupling coefficient hasbeen changed. Thus, it becomes possible to provide a reliable opticalinterconnection system.

According to the sixteenth aspect of the present invention, in whichthere is provided an optical communication system that uses thesurface-emission laser diode according to any of the first throughthirteenth aspects of the present invention or the surface-emissionlaser array according to the fourteenth aspect of the present invention,it becomes possible to provide a reliable optical communication system.

With the surface-emission laser diode or surface-emission laser array ofthe present invention, it should be noted that high output power isobtained while maintaining the fundamental transverse mode oscillation.Thus, high coupling efficiency is obtained with regard to an opticalfiber. Further, because the laser oscillation of higher-order transversemode is suppressed effectively, there occurs little change of opticalcoupling coefficient even when the operational state of the laser diodesuch as the optical output power is changed, and thus, there occurslittle change in the injection of optical signals into the opticalfiber. In addition, a larger output power is obtained as compared withthe conventional art, while this enables long-distance communication.Thus, the present invention can provide a reliable communication system.

According to the seventeenth aspect of the present invention, in whichthere is provided an electro-photographic system using any of thesurface-emission laser diode according to any of the first throughthirteenth aspect of the present invention or the surface-emission laserarray according to the fourteenth aspect of the present invention, itbecomes possible to provide a low-cost and high-definitionelectro-photographic system.

Conventionally, it has been difficult to use a surface-emission laserdiode for the optical source of the writing system of anelectro-photographic system because of its small output power. On theother hand, the surface-emission laser diode or surface-emission laserarray of the present invention can provide a high output power whilemaintaining the fundamental transverse mode. Thus, with the presentinvention, it becomes possible to use a surface-emission laser diode forthe writing optical source of an electro-photographic system.

With the use of a surface-emission laser diode for the writing opticalsource of an electro-photographic system, an output optical beam havingof circular cross-section is obtained, while shaping of optical beam isfacilitated with the use of such an optical beam of the circularcross-section.

Further, because of high positional precision between the laser diodesin the array, it becomes possible to focus plural optical beams withexcellent reproducibility while using a single lens. Thereby, theconstruction of the optical system is simplified, and it becomespossible to obtain a high-definition system with low cost. Because oflarge output power, the surface-emission laser diode of the presentinvention enables high-speed writing particularly in the case it is usedin the form of an array.

Thus, with the present invention, it becomes possible to provide alow-cost and high-definition electro-photographic system.

According to an eighteenth aspect of the present invention, in whichthere is provided an optical disk system that uses the surface-emissionlaser diode according to any of the first through thirteenth aspect ofthe present invention or the surface-emission laser array according tothe fourteenth aspect of the present invention as the reading/writingoptical source, it becomes possible to provide an optical disk system ofhigh reliability and capable of performing high-speed disk access(high-speed reading and writing).

Conventionally, it has been difficult to use a surface-emission laserdiode for the optical source of an optical disk system because of itssmall output power. On the other hand, the surface-emission laser diodeor surface-emission laser array of the present invention can provide ahigh output power while maintaining the fundamental transverse mode.Thus, with the present invention, it becomes possible to use asurface-emission laser diode for the optical source of an optical disksystem. Further, it becomes possible to construct a reliable opticaldisk system.

Particularly, as a result of use of surface-emission laser array, itbecomes possible to perform high-density reading and writing, while thisenables to construct a high-speed optical disk system.

Thus, according to the present invention, it becomes possible to providea highly reliable optical disk system capable of performing high-speedaccess (high-speed reading and writing).

According to the nineteenth through twenty second aspects of the presentinvention, in which there is provided a surface-emission laser diodeconstructed on a substrate having a substrate surface, comprising: anactive layer parallel to said substrate surface; a pair of cavity spacerlayers provided so as to sandwich said active layer; a pair ofdistributed Bragg reflectors provided across said active layer and saidcavity spacer layers so as to sandwich said active layer and said cavityspacer layers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein any of a width of said low refractive index core or ashape of said periodic structure is changed between a specific directionparallel to said substrate surface and other directions parallel to saidsubstrate surface different from said specific direction, it becomespossible to control the polarization direction of the output laser beamstably in any desired direction, while causing the laser diode tooscillate in the single fundamental transverse mode up to high outputpower state. Further, such a surface-emission laser diode can bemodulated at high speed.

Thus, with the surface-emission laser diode of the nineteenth throughtwenty second aspects of the present invention, the polarizationdirection of the output laser beam is aligned in an arbitrarily selecteddesired direction. Further, it becomes possible to maintain singlefundamental transverse mode laser oscillation up to high output powerstate. This means that high output power is obtained while maintainingthe single fundamental transverse mode oscillation.

Further, the surface-emission laser diode of the present invention ischaracterized by low device resistance, while this means that the laserdiode of the present invention has excellent frequency characteristics.Further, heat generation is reduced with the laser diode of the presentinvention, while this means that the laser diode can provide highdifferential gain and high output power. Thus, it becomes possible toachieve a high relaxation oscillation frequency. Thereby, asurface-emission laser diode capable of high speed modulation isobtained.

Particularly, with the surface-emission laser diode of the twenty secondaspect of the present invention, it becomes possible to produce a laserbeam having a high polarization ratio and having a polarizationdirection in a specific direction while maintaining the singletransverse mode oscillation up to high output power state.

According to the twenty third aspect of the present invention, in whichthere is provided a surface-emission laser diode constructed on asubstrate having a substrate surface, comprising: an active layerparallel to said substrate surface; a pair of cavity spacer layersprovided so as to sandwich said active layer; a pair of distributedBragg reflectors provided across said active layer and said cavityspacer layers so as to sandwich said active layer and said cavity spacerlayers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein said periodic structure is provided partially in saidplane parallel to said substrate surface in a specific direction, thepolarization direction is controlled stably in a desired specificdirection. Further, high output power operation is realized whilemaintaining single fundamental transverse mode laser oscillation.Further, it becomes possible modulate at high speed with the laser diodeof the twenty third aspect of the present invention.

Thus, with the surface-emission laser diode of the twenty third aspectof the present invention, too, the polarization direction of the laserbeam is aligned in a desired specific direction. Further, it becomespossible to obtain the single fundamental transverse mode oscillation upto high output power state. Further, because of the low deviceresistance, the surface-emission laser diode of the present inventionhas excellent electric characteristics, particularly the frequencycharacteristics. Because of low heat generation and high efficiency ofheat radiation, it becomes possible with the surface-emission laserdiode of the twenty aspect of the present invention to obtain highdifferential gain and high output power. Further, it becomes possible toobtain high relaxation oscillation frequency.

Thus, with the twenty third aspect of the present invention, it becomespossible to obtain a surface-emission laser diode capable of high speedmodulation.

According to the twenty fourth aspect of the present invention, in whichthere is provided a surface-emission laser diode according to any of thenineteenth through twenty third aspects of the present invention,wherein said active layer is formed of a III-V semiconductor material,said active layer containing any or both of Ga and In as a group IIIelement, said active layer containing any or all of As, N, Sb and P as agroup V element, the polarization direction is controlled stably in adesired specific direction. Further, high output power operation isrealized while maintaining single fundamental transverse mode laseroscillation. Further, it becomes possible modulate at high speed withthe laser diode of the twenty third aspect of the present invention.Particularly, it becomes possible to provide a surface-emission laserdiode of long wavelength band having improved temperaturecharacteristics.

More specifically, with the twenty fourth aspect of the presentinvention, it becomes possible to construct a surface-emission laserdiode having an oscillation wavelength between 1.1 μm and 1.6 μm on aGaAs substrate. With the use of the GaAs substrate, it becomes possibleto provide excellent distributed Bragg reflector by using the AlGaAsmixed crystal, and the laser diode having such a construction showssuperior characteristics.

Particularly, with the use of GaInNAs, in which small amount ofnitrogen, typically several percent or less in concentration, is addedto GaInAs, for the active layer of the laser diode, a large banddiscontinuity is secured with regard to the GaAs barrier layer in theconduction band, and it becomes possible to provide a device havingexcellent temperature characteristics over the device of the samewavelength band and constructed on a conventional InP substrate.

Further, similarly with the nineteenth through twenty third aspects ofthe present invention, it becomes possible to set the polarizationdirection stably in an arbitrary direction with the laser diode of thepresent invention. Further, with the twenty fourth aspect of the presentinvention, it becomes possible to provide a surface-emission laser diodecapable of maintaining single fundamental transverse mode laseroscillation up to high output power state and capable of performing highspeed modulation. Thus, the laser diode of the twenty fourth aspect ofthe present invention is particularly suitable for optical fibercommunication.

According to the twenty fifth aspect of the present invention, in whichthere is provided a surface-emission laser array formed of asurface-emission laser diode according to any of the nineteenth throughtwenty fourth aspects of the present invention, it becomes possible tocontrol the polarization direction in an arbitrarily chosen specificdirection stably. Further, with the surface-emission laser array of thetwenty fifth aspect of the present invention, it becomes possible toprovide a monolithic surface-emission laser array providing high putpower in the single fundamental transverse mode oscillation and capableof performing high speed modulation.

Thus, with the twenty fifth aspect of the present invention, it becomespossible to provide a surface-emission monolithic laser array having apolarization direction aligned in an arbitrarily chosen specificdirection and is capable of oscillating in the fundamental transversemode up to high output power while providing high quality beam and isfurther capable of high speed modulation.

According to the twenty sixth aspect of the present invention, in whichthere is provided an optical interconnection system that uses thesurface-emission laser diode according to any of the nineteenth throughtwenty fourth aspects of the present invention or the surface-emissionlaser array according to the twenty fifth aspect of the presentinvention, it becomes possible to provide a reliable opticalinterconnection system capable of high speed transmission by using thesurface emission laser diode or surface emission laser array in whichthe polarization direction is controlled stably in an arbitrarily chosenspecific direction and high output power is obtained while maintainingthe fundamental transverse mode (high output power is obtained in thesingle fundamental transverse mode oscillation) and is capable ofperforming high speed modulation, for the optical source.

According to the twenty seventh aspect of the present invention, inwhich there is provided an optical communication system that uses thesurface-emission laser diode according to any of the nineteenth throughtwenty fourth aspects of the present invention or the surface-emissionlaser array according to the twenty fifth aspect of the presentinvention, it becomes possible to provide a reliable opticalcommunication system suitable for optical fiber communication andcapable of high speed transmission by using the surface emission laserdiode or surface emission laser array in which the polarizationdirection is controlled stably in an arbitrarily chosen specificdirection and high output power is obtained while maintaining thefundamental transverse mode (high output power is obtained in the singlefundamental transverse mode oscillation) and is capable of performinghigh speed modulation, for the optical source.

According to the twenty eighth aspect of the present invention, in whichthere is provided an electrophotographic system that uses thesurface-emission laser diode of any of the nineteenth through twentyfourth aspects of the present invention or the surface-emission laserarray according to the twenty fifth aspect of the present invention, itbecomes possible to provide a low cost electrophotographic systemcapable of high speed writing with high definition, by using the surfaceemission laser diode or surface emission laser array in which thepolarization direction is controlled stably in an arbitrarily chosenspecific direction and high output power is obtained while maintainingthe fundamental transverse mode (high output power is obtained in the issingle fundamental transverse mode oscillation) and is capable ofperforming high speed modulation, for the optical source.

According to the twenty ninth aspect of the present invention, in whichthere is provided an optical disk system that uses a surface emissionlaser diode according to any of the nineteenth through twenty fourthaspects of the present invention or the surface-emission layer arrayaccording to the twenty ninth aspect of the present invention, itbecomes possible to provide a reliable optical disk system capable ofhigh speed access, by using the surface emission laser diode or surfaceemission laser array in which the polarization direction is controlledstably in an arbitrarily chosen specific direction and high output poweris obtained while maintaining the fundamental transverse mode (highoutput power is obtained in the single fundamental transverse modeoscillation) and is capable of performing high speed modulation, for theoptical source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the surface-emission laser element ofExample 1;

FIG. 2 is a diagram showing a part of the region of FIG. 1 in whichoxygen ion implantation is made in detail;

FIGS. 3A and 3B are diagrams showing the mode distribution for the casein which there is no antiguiding structure and in which there isprovided an antiguiding structure;

FIG. 4 is a diagram explaining the surface-emission laser element ofExample 2;

FIG. 5 is a diagram explaining the surface-emission laser element ofExample 2;

FIG. 6 is a diagram explaining the surface-emission laser element ofExample 3;

FIG. 7 is a diagram showing a different construction of the oxygen ionimplantation region of the surface-emission laser element of Example 1;

FIG. 8 is a diagram showing the surface-emission laser element ofExample 4;

FIG. 9 is a diagram showing the surface-emission laser element ofExample 5;

FIG. 10 is a diagram showing an example in which the antiguidingstructure is provided inside an n-type distributed Bragg reflector inthe surface-emission laser element of FIG. 9 similarly to the case ofExample 4;

FIGS. 11A and 11B are diagrams showing the surface-emission laserelement of Example 6;

FIGS. 12A and 12B are diagrams showing the surface-emission laserelement of Example 7;

FIGS. 13A and 13B are diagrams showing the surface-emission laserelement of Example 8;

FIGS. 14A and 14B are diagrams showing the surface-emission laserelement of Example 9;

FIG. 15 is a diagram showing the surface-emission laser array of Example10;

FIG. 16 is a diagram showing an example of the surface-emission lasermodule;

FIG. 17 is a diagram showing an example of the parallel opticalinterconnection system connecting between devices;

FIG. 18 is a diagram showing an optical telecommunication system ofExample 12;

FIG. 19 is a diagram showing an electro-graphic system of Example 13;

FIG. 20 is a diagram showing the optical disk system of Example 14;

FIGS. 21A-21C are diagrams showing the surface-emission laser diode ofExample 15;

FIG. 22 is a diagram showing a modification of the surface-emissionlaser diode of FIGS. 21A-21C;

FIGS. 23A-23C are diagrams showing the surface-emission laser diode ofExample 16;

FIG. 24 is a diagram showing a modification of the surface-emissionlaser diode of FIGS. 23A-23C;

FIGS. 25A-25C are diagrams showing the surface-emission laser diode ofExample 17;

FIG. 26 is a plan view (top plan view) showing a surface-emission laserarray of Example 18;

FIG. 27 is a diagram showing the laser array module as an example of theoptical interconnection system of Example 19;

FIG. 28 is a diagram showing a parallel optical interconnection systemconnecting between devices as an example of the optical interconnectionsystem of Example 19;

FIG. 29 is a diagram showing an optical LAN system as an example of anoptical telecommunication system of Example 20;

FIG. 30 is a diagram showing an electro-graphic system of Example 21;

FIG. 31 is a diagram showing the optical disk system of Example 22.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be explained for the best modefor implementing the invention.

(First Mode of Invention)

According to the first mode of the present invention, there is provideda surface-emission laser diode, comprising:

-   -   an active layer;    -   a pair of cavity spacer layers formed at both sides of said        active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across a structure formed of said active layer and said cavity        spacer layers,    -   said current confinement structure being formed by a selective        oxidation process of a semiconductor layer,    -   said pair of distributed Bragg reflectors being formed of        semiconductor materials,    -   wherein there is provided a region containing an oxide of Al and        having a relatively low refractive index as compared with a        surrounding region in any of said semiconductor distributed        Bragg reflector or said cavity spacer layer in correspondence to        a part spatially overlapping with said current injection region        in a laser cavity direction.

In the prior art surface-emission laser diodes, it should be noted thatthe distributed Bragg reflector above the Al oxide layer has been formedby a dielectric material. Because a dielectric material is an insulator,it is not possible to feed electric current through the distributedBragg reflector as long as the distributed Bragg reflector is formed byusing a dielectric material.

Thus, with the surface-emission laser diode that uses a dielectricdistributed Bragg reflector, it is inevitable to use a structure such asintra-cavity contact in which the current injection to the active regionis made via a contact layer provided inside the distributed Braggreflector or in the interior of the device as in the prior artsurface-emission laser diode that has the antiguiding structure.

In the case of using such an intra-cavity contact structure, in whichthe current injection is made from the peripheral part of the device viaa thin contact layer, there generally arises a problem that the deviceresistance is increased. Thus, with the laser diode having a dielectricdistributed Bragg reflector, a restriction is imposed upon the currentinjection path, and there arises a problem that the device resistance isincreased.

Contrary to this, the distributed Bragg reflector is constructed, withthe first mode of the present invention, by a semiconductor materialincluding the part that includes the low refractive index region formedof Al oxide as the antiguiding structure.

Thus, with the present invention, it becomes possible to carry outcurrent injection from the contact layer provided at the uppermost partof the device structure in the direction toward the substrate, similarlyto the case of conventional surface-emission laser diode of theselective oxidation type. Thereby, it becomes possible to achievecurrent injection, even in the case there exists an insulation region ofAl oxide at the central part of the device, via the surrounding regionof relatively large area, and the increase of resistance of the deviceis effectively prevented.

Thus, the present invention enables current injection via thedistributed Bragg reflector, and with this, the effect of deviceresistance can be reduced as compared with the conventional antiguidingsurface-emission laser diode. Thereby, a large degree of freedom ofdevice is achieved.

With the distributed Bragg reflector, in which it is required to controlthe film thickness precisely, there is a demand for precise control ofdeposition rate at the time of formation of the distributed Braggreflector. With regard to this aspect, it should be noted that use oftwo different deposition apparatuses at the time of forming thedistributed Bragg reflector in correspondence to the use of differentmaterials and different deposition processes not only complicates themanufacturing process but also increases the cost of the device.

Contrary to this, the first mode of the present invention can form thedistributed Bragg reflector in a single crystal growth apparatus of asemiconductor material such as an MOCVD apparatus collectively, and withthis, the fabrication process of the laser diode is simplified.

Thus, with the first mode of the present invention, it becomes possibleto provide a selectively oxidized surface-emission laser diode having anantiguiding structure, in which the manufacturing cost is suppressed,the freedom of design is increased and the device resistance is reduced.

(Second Mode of Invention)

According to the second mode of the present invention, there is provideda surface-emission laser diode, comprising:

-   -   an active layer;    -   a pair of cavity spacer layers formed at both sides of said        active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across a structure formed of said active layer and said pair of        cavity spacer layers,    -   said current confinement structure comprising a high resistance        region formed by an ion implantation process,    -   said pair of distributed Bragg reflectors being formed of        semiconductor materials,    -   wherein there is provided a region containing an oxide of Al and        having a relatively low refractive index than a surrounding        region in any of said semiconductor distributed Bragg reflector        or said cavity spacer layer in correspondence to a part        spatially overlapping with said current injection region in a        laser cavity direction.

As explained with regard to the first mode of the invention, the priorart antiguiding surface-emission laser diode uses a distributed Braggreflector of a dielectric material for the upper distributed Braggreflector provided above the low refractive index region of Al oxideforming the antiguiding structure, while such a structure prohibitsinjection of drive current from the top part of the distributed Braggreflector formed of the dielectric material.

Contrary to this, the surface-emission laser diode of the second modeuses a semiconductor distributed Bragg reflector formed of asemiconductor material including the part in which the low refractiveindex region of Al oxide is formed. Thereby, it becomes possible to feeda drive current through the distributed Bragg reflector.

With regard to the conventional surface-emission laser diode of hydrogenion implantation type, in which there is formed a region of highresistance by ion implantation process of hydrogen after formation ofthe device structure by using semiconductor materials, such a devicedoes have the advantage that the device can be formed very simplywithout conducting an etching process in the crystal growth part. In theprior art antiguiding surface-emission laser diode that uses thedielectric mirror, it should be noted that there is a need of forming anelectrode, or the like under the dielectric mirror for currentinjection. Thus, it has been difficult to use such a simpleconstruction.

Further, with the prior art antiguiding surface-emission laser diode, ithas been necessary to use the intra-cavity contact construction in orderto allow current injection from the bottom part of the dielectricmirror, while such a construction raises the problem of increased deviceresistance as explained before. Further, from the construction of theprior art antiguiding surface-emission laser diode that uses theselective oxidation process, it is difficult to drive the constructionof the hydrogen ion implantation type surface-emission laser diodehaving the antiguiding structure.

Thus, according to the second mode of the present invention, in whichthe distributed Bragg reflector or the cavity spacer layer includingtherein the Al oxide is formed of a semiconductor material, it becomespossible to inject current from the top part of the distributed Braggreflector similarly to the case of the first mode invention. Thereby, itbecomes possible to obtain a device having an antiguiding structure witha very simple construction, similarly to the case of the conventionalhydrogen-ion injected surface-emission laser diode. Further, because itis possible to inject drive current from the top part of the device, thelaser diode of the present mode can avoid the problem of increase ofdevice resistance, similarly to the first mode of the present invention.

Thus, with the present embodiment, it becomes possible to provide ahydrogen ion implantation type surface-emission laser diode having anantiguiding structure wherein the degree of freedom of design isincreased and the device resistance is decreased.

(Third Mode of Invention)

According to the third mode of the present invention, there is provideda surface-emission laser diode, comprising:

-   -   an active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across said active layer;    -   said current confinement structure being formed of a selective        oxidation process of a semiconductor material,    -   said surface-emission laser diode comprising an AlGaAs mixed        crystal layer,    -   a part of said AlGaAs mixed crystal layer having spatial        overlapping with said current injection region in a laser cavity        direction,    -   said part of said AlGaAs mixed crystal being formed by selective        ion implantation of molecules containing oxygen and a thermal        annealing process following said selective ion implantation        process, said part of said AlGaAs mixed crystal having a        relatively low refractive index as compared with a surrounding        region wherein ion implantation of molecules containing oxygen        is not made and located on an identical plane of said part, said        plane being perpendicular to said laser cavity direction.

According to the construction of the third mode of the presentinvention, the refractive index of the AlGaAs layer is decreased ascompared with the surrounding region in the part in which the ionimplantation of the molecules containing oxygen has been made as aresult of oxidation caused by the thermal annealing process, and withthis, there is formed an antiguiding structure in a part of the cavityinterior.

Thus, with the present mode of the invention, it becomes possible toform the antiguiding structure at any desired part inside the devicestructure by using the selective oxidation process conducted by way ofion implantation of molecules containing oxygen followed by thermalannealing process, in place of conventional selective oxidationconducted in a high temperature ambient containing water vapor.

Because it is not necessary to expose the surface of the AlGaAs layer tothe high-temperature water vapor ambient in the present mode of theinvention, contrary to the prior art, there occurs no formation of oxidelayer on the device surface, and it becomes possible to resume thecrystal growth of the semiconductor layers. Thereby, the degree offreedom of design is increased, and a device structure superior insuppressing the higher-order transverse mode laser oscillation can beobtained.

Particularly, as a result of formation of the antiguiding structure atthe central part of the cavity structure with limited extent, it becomespossible to decrease the spatial overlapping of the higher-order modedistribution with the gain region (current injection region) of theactive layer, and thus, it becomes possible to suppress the laseroscillation in the higher-order transverse mode.

FIGS. 3A and 3B explain the foregoing situation.

Referring to the drawings, FIG. 3A shows the mode distribution for thecase there is no antiguiding structure, while FIG. 3B shows the modedistribution in the case the antiguiding structure is provided at thecentral part of the device with a limited extent. Thereby, it should benoted that the upper graphs of FIGS. 3A and 3B represent thedistribution of the fundamental transverse mode, while the lower graphsof FIGS. 3A and 3B represent the distribution of the first-ordertransverse mode. Further, it should be noted that FIGS. 3A and 3Bincludes the representation of the gain region formed in correspondenceto the current injection.

In the case of the fundamental transverse mode having a large modedistribution at the mesa central part, it is difficult to modify themode distribution profile toward the peripheral part of the mesastructure. On the other hand, in the case of the higher-order transversemode, which has the electric field strength of zero at the mesa centerand a large mode distribution at the peripheral part, it becomespossible to decrease the mode distribution (electric field distribution)at the mesa central part by modifying the mode distribution toward theperipheral direction of the mesa structure by providing the antiguidingstructure.

Thus, with the present invention, the gain for the higher-ordertransverse mode is suppressed, and the laser diode can perform thefundamental transverse mode operation up to the high output power state.

Further, with regard to the fundamental transverse mode, which is not somuch influenced by the antiguiding structure as in the case of thehigher transverse mode, there nevertheless is caused a broadening of thedistribution profile with the formation of the antiguiding structure asshown in FIG. 3B, and thus, the electric field strength distributionbecomes more broad about the center of the mode. Thereby, the peakelectric field strength in the gain region is decreased, and occurrenceof spatial hole-burning can be reduced. Further, because thehigher-order transverse mode is spatially separated from the gain regionby the antiguiding structure with sufficient distance, it becomespossible to user a larger diameter for the current confinement structureas compared with the conventional case. Thereby, the device resistanceis reduced further and more higher output power operation becomespossible.

(Fourth Mode of Invention)

According to the fourth mode of the present invention, there is provideda surface-emission laser diode, comprising:

-   -   an active layer;    -   a current confinement structure defining a current injection        region into said active layer; and    -   a pair of distributed Bragg reflectors opposing with each other        across said active layer,    -   said current confinement structure comprising a high resistance        region formed by an ion implantation process,    -   said surface-emission laser diode comprising an AlGaAs mixed        crystal layer,    -   a part of said AlGaAs mixed crystal layer having a spatial        overlapping with said-current injection region in a laser cavity        direction,    -   said part of said AlGaAs mixed crystal spatially overlapping        with said current injection region being processed by an ion        implantation process of molecules containing oxygen and a        thermal annealing process following said ion implantation        process and having a relatively low refractive index as compared        with a surrounding region not injected with molecules containing        oxygen and located on an identical plane of said part, said        plane being perpendicular to said laser cavity direction.

With the fourth mode, too, the AlGaAs mixed crystal is oxidized incorrespondence to the region where the ion implantation of the moleculescontaining oxygen has been made as a result of the thermal annealingprocess, and the refractive index is decreased as compared with thesurrounding region. Thereby, the antiguiding structure is formed insidethe cavity structure.

Thus, by forming the antiguiding structure as a result of selectiveoxidation conducted by ion implantation of molecules containing oxygenfollowed is by thermal annealing process, in place of the conventionalselective oxidation process conducted in the high-temperature ambientcontaining water vapor, it becomes possible with the fourth mode of thepresent invention to form the antiguiding structure at any desired partof the device structure.

Further, in view of no need of exposing the surface of the AlGaAs layerto the high-temperature water vapor ambient for the propose of oxidationwith the present mode of the invention, there is caused no formation ofoxide film at the device surface, while this allows regrowth ofsemiconductor layers on such a device surface. Thereby, the degree offreedom of design is increased with the fourth mode of the presentinvention, and a device structure capable of effectively suppressinglaser oscillation in the higher-order transverse mode is realized.

With the hydrogen ion implantation type surface-emission laser diodethat lacks the built-in waveguide structure, it is easy to expand thehigher-transverse mode toward the peripheral direction of the mesastructure by providing the antiguiding structure as explained withreference to FIG. 3B, and with this, it becomes possible to decrease theamplitude of the higher-order transverse mode (electric field strength)at the central part of the mesa structure.

As a result, spatial overlapping of the higher-order transverse modewith the gain region (current injection region) in the active layer isreduced, and laser oscillation at the higher-order transverse mode iseffectively suppressed. In a surface-emission laser diode having thehydrogen ion implantation current confinement structure that lacks thebuilt-in waveguide structure, it is known that the transverse mode isunstable with regard to the change of the driving condition, while theuse of the antiguiding structure as in the case of the present mode ofthe invention effectively suppresses the occurrence of laser oscillationa the higher-order transverse mode.

Further, with regard to the fundamental transverse mode, it should benoted that the peak of the mode is suppressed as represented in FIG. 3Band the distribution of the electric field is broadened at the modecentral part. As a result of such decrease of the peak electric fieldstrength in the gain region, it becomes possible with the present modeof the invention to reduce the occurrence of spatial hole-burning.

(Fifth Mode of Invention)

In the fifth mode of the present invention, there is provided asurface-emission laser diode according to any of the first throughfourth modes, wherein there is provided any of a GaAs layer or a GaInPmixed crystal layer adjacent with said AlGaAs mixed crystal layerconstituting said region of relatively low refractive index with respectto said surrounding region not injected with said molecules containingoxygen and located on said identical plane of said part where said ionimplantation of said molecules has been made.

It should be noted that there occurs no substantial oxidation in thesemiconductor layer of GaInP or GaAs even when oxygen ion implantationhas been made. Thus, in the case the distribution profile of the oxygenions in the AlGaAs layer is broad, it becomes possible to prevent theunwanted formation of oxide layer outside the desired region, byproviding a GaInP layer or a GaAs layer or a semiconductor structureformed of any of these adjacent to the AlGaAs layer to be selectivelyoxidized as set forth in the fifth mode of the present invention.Thereby, adjustment of resonance condition is achieved easily.

(Sixth Mode of Invention)

According to the sixth mode of the present invention, there is provideda surface emission laser diode according to any of the first throughfifth modes of the present invention, wherein said AlGaAs mixed crystallayer, constituting said region of relatively low refractive index withrespect to said surrounding region not made with ion implantation ofmolecules containing oxygen and located on said identical planeperpendicular to said laser cavity direction, is provided at a locationcorresponding to an anti-node of a standing wave of laser oscillationoccurring in said cavity structure, said AlGaAs mixed crystal layerbeing doped to a higher concentration level as compared with otherAlGaAs mixed crystal layers constituting said surface-emission laserdiode.

By using the construction of the sixth mode of the present invention,the region where ion implantation of the molecules containing oxygen hasbeen made and thermal annealing process is made subsequently forms anAlOx layer as a result of the oxidation. Thereby, such a region becomestransparent to the fundamental transverse mode optical radiation, andoptical loss of the fundamental transverse mode is eliminated.

On the other hand, the region not made with the ion implantation of themolecules containing oxygen absorbs the higher-order transverse modeoptical radiation having a large mode amplitude in such a region becauseof the high concentration doping made thereto. Thereby, laseroscillation at the higher-order transverse mode is effectivelysuppressed. Particularly, it should be noted that there occurs a largemode distribution (electric field intensity) in correspondence to theanti-node in the optical cavity. Thus, particularly large absorptionloss is caused in the higher-order transverse mode in such a part, andlaser oscillation at the higher-order transverse mode is suppressedeffectively.

(Seventh Mode of Invention)

According to the seventh mode of the present invention, there isprovided a surface-emission laser diode according to any of said firstthrough sixth modes of the present invention, wherein said region ofrelatively low refractive index is provided in plural numbers.

By providing the anti-guiding structure having a relatively lowrefractive index as compared with the surrounding region with pluralnumbers as set forth in the seventh mode of the present invention, theantiguiding action is adjusted easily, and the degree of freedom ofdesign is increased.

Further, by providing the antiguiding structure in plural numbers, alarge antiguiding action is obtained, and the efficiency of suppressingthe higher-order transverse mode laser oscillation is increased.

(Eighth Mode of Invention)

According to the eighth mode of the present invention, there is provideda surface-emission laser diode according to any of the first throughseventh modes of the present invention, wherein said region ofrelatively low refractive index is provided inside an n-type distributedBragg reflector constituting one of said pair of distributed Braggreflectors.

According to the construction of the eighth mode of the presentinvention, it becomes possible to suppress the higher-order transversemode laser oscillation without increasing the device resistance, byproviding the antiguiding structure of low refractive index regionhaving a refractive index lower than the surrounding region in then-type distributed Bragg reflector.

(Ninth Mode of Invention)

According to the ninth mode of the present invention, there is provideda surface-emission laser diode according to any of the first througheighth modes of the present invention, wherein there is further provideda region of relatively high refractive index around said region ofrelatively low refractive index provided in spatial overlapping withsaid laser cavity region in said laser cavity direction, and whereinthere is further provided a cladding region of low refractive index withrespect to said high refractive region such that said cladding region islocated around said region of high refractive index.

With the antiguiding surface-emitting laser diode of the prior art inwhich the low refractive index region is provided in the region wherethe laser oscillation takes place, there inevitably arises a tendency,due to the nature of the waveguide structure, that optical loss(diffraction loss) occurs as a result of mode leakage in the directionperpendicular to the direction of laser oscillation. Thereby, there hasbeen caused problems such as increase of threshold current, decrease ofoutput, or the like.

In order to overcome this problem, the Non-Patent Reference 5 proposes astructure of surface-emission laser diode called S-ARROW.

With this conventional art, a region having a different effectiverefractive index is formed in the direction parallel to the substratesurface with adjustment of the film thickness by an etching process,such that there is formed a region of high refractive index surroundingthe region of low refractive index, the laser oscillation region beinglocated at the center of the low refractive index region. Further, thereis provided another low refractive index region outside the highrefractive index region so as to surround the high refractive indexregion.

In this way, by providing a high refractive index region and another lowrefractive index region consecutively around the low refractive indexregion including the laser oscillation region at the central partthereof, there is formed an antiguiding structure by the low refractiveindex region that includes the laser oscillation region and the highrefractive index region surrounding the low refractive index region.Further, the low refractive index region provided around the highrefractive index region functions similarly to a cladding layer andconfines the transverse mode optical radiation. Thus, with such aconstruction, the leakage loss in the direction parallel to thesubstrate caused by diffraction can be suppressed.

Because the optical radiation is tend to be confined in the highrefractive index region, there tends to occur a remarkable concentrationof higher-order transverse mode in such a high refractive index regionprovided between the cladding layer and the low refractive index regionincluding the laser oscillation region at the central part thereof, andit becomes possible to reduce the coupling between the gain region andthe higher-order mode optical radiation. Thereby, laser oscillation withhigher-order transverse mode is effectively suppressed.

Further, the use of the construction of the ninth mode of the presentinvention with the surface-emission laser diode of any of the firstthrough eighth modes of the present invention, in which the antiguidingstructure is formed by forming a low refractive index region of Al oxidein spatial overlapping with the laser cavity region, enables to realizea large refractive index difference as compared with the prior art, inview of the fact that the oxide of Al has a small refractive index ofabout one-half of the refractive index of the semiconductor material,and large antiguiding is easily attained. Thus, the coupling between thehigher-order transverse mode and the gain region is suppressed further,and with this, the laser oscillation with the higher-order transversemode can be suppressed further.

Further, in the case the cladding region is formed of a low refractiveindex region that contains an oxide of Al, a large optical confinementof the transverse mode is achieved. Thereby, optical loss caused by modeleakage is reduced significantly.

Further, in the case the selective oxidation caused by ion implantationof molecules or ions containing oxygen and subsequent thermal annealingprocess is used for the formation of the antiguiding structure as in thecase of the first through sixth modes of the present invention, itbecomes possible to fabricate the device without conducting processessuch as etching or regrowth, and the fabrication process of the laserdiode is significantly simplified.

With the case of a surface-emission laser diode of the hydrogen ionimplantation type, a particularly large effect can be attained as willbe explained below.

In the case of the surface-emission laser diode of the hydrogen ionimplantation type, there is provided no confinement structure for thetransverse mode. Thus, there arises a problem in that the loss caused bymode leakage becomes very large in the case an antiguiding structure isprovided. With the surface-emission laser diode of the ninth mode of theinvention, it becomes possible to decrease the loss by mode leakagesignificantly, y providing a low refractive index cladding of Al oxideoutside the laser oscillation region.

With regard to the cladding region for confinement of the transversemode, it is also possible to use a periodic structure in which a lowrefractive index region and a high refractive index region are repeatedalternately. By choosing the width of the respective regions, with sucha periodic structure of refractive index, to be equal to an odd numbermultiple of ¼ wavelength in the plane perpendicular to the laser cavitydirection of the fundamental transverse optical radiation, it becomespossible to form an optical confinement structure (cavity) similar tothe distributed Bragg reflector provided in the laser cavity direction.Thereby, by adjusting the width of the reflection wavelength band byappropriately setting the repetition period, it becomes possible torealize a very high optical confinement effect selectively with regardto the fundamental transverse mod optical radiation.

Thus, with the ninth mode of the present invention, mode leakage in thedirection perpendicular to the laser cavity direction is reduced, andthe laser oscillation with higher-transverse mode is suppressed furthereffectively, and it becomes possible to obtain a single fundamentaltransverse mode laser oscillation up to further high output power.

(Tenth Mode of Invention)

According to a tenth mode of the present invention, there is providedanisotropy in the width of the high refractive index region surroundedby the cladding region in the surface-emission laser diode of the ninthmode of the present invention.

With the tenth mode of the present invention, in which there is providedan anisotropy in the width of the region surrounded by the claddingregion in the direction perpendicular to the laser cavity direction, thepolarization direction of the laser beam emitted from the laser diode iscontrolled in a specific direction, and the single fundamentaltransverse mode laser oscillation can be maintained up to further higheroptical output.

By providing such a high refractive index region and low refractiveindex region around the low refractive index region that includes thelaser oscillation region at the central part thereof, the low refractiveindex region outside the high refractive index region functions as acladding layer that confines the transverse mode. With this, the opticalloss caused by mode leakage can be reduced. Further, there occurs aconcentration of higher-order transverse mode in the high refractiveindex region provided between the cladding layer and the low refractiveindex region that includes the laser oscillation region at the centralpart thereof, and coupling between the gain region and the higher-ordertransverse mode is reduced. With this, it becomes possible to suppressthe laser oscillation with higher-transverse mode.

Further, by providing anisotropic shape such as rectangular orelliptical shape to the width of the region surrounded by the lowrefractive index cladding layer as in the case of the tenth mode of theinvention, there occurs anisotropy in the lateral optical confinement,and it becomes possible to change the transverse mode distributionprofile between directions of different anisotropy. Thereby, byadjusting the width of the region surrounded by the low refractive indexcladding and also the effective refractive index of the cladding region,the high refractive index region and the low refractive index core, itbecomes possible to realize a large coupling for a fundamentaltransverse mode distribution with the gain region in a specificdirection. Thereby, it becomes possible to cause laser oscillation inthe fundamental transverse mode with a specific polarization direction.

Incidentally, it should be noted that higher-order transverse modegenerally has very small electric field amplitude and is thussusceptible to the influence of the antiguiding structure. Thus,coupling with the gain regions is inherently small, and laseroscillation with such higher-order transverse mode can be effectivelyand sufficiently suppressed by providing such an antiguiding structure.

From above, it becomes possible with the tenth mode of the presentinvention to provide a surface-emission laser diode capable ofcontrolling the polarization direction and capable of maintaining thesingle fundamental transverse mode operation up to high output power.

(Eleventh Mode of Invention)

According to the eleventh mode of the present invention, there isprovided a surface-emission laser diode according to the ninth mode ofthe present invention in which the foregoing cladding region is providedwith a pair so as to oppose with each other across the laser cavityregion in a specific direction perpendicular to the laser cavitydirection.

With the eleventh mode of the surface-emission laser diode, the lowrefractive index region of Al oxide outside the region that includes thelaser oscillating region at the central part thereof is now provided soas to form a pair opposing with each other across the foregoing lasercavity region in only one specific direction. With this, thepolarization direction is controlled to a specific direction and thelaser diode maintains the single fundamental transverse mode operationup to high output power.

As explained before, there is a tendency that there occurs optical lossas a result of mode leakage in the lateral direction with the laserdiode having such an antiguiding structure. In order to reduce the modeleakage loss, it is effective to provide a low refractive index claddingas in the case of the surface-emission laser diode of the ninth mode ofthe invention.

Thus, by providing the low refractive index cladding layer only in aspecific direction so as to sandwich the laser oscillation regionlaterally, it becomes possible to reduce the mode leakage loss in thedirection in which the low refractive index cladding regions areprovided, and it becomes possible to achieve laser oscillationselectively with the fundamental transverse mode that has an electricfield component (polarization) in this direction.

In addition, the surface-emission laser diode of the hydrogen ionimplantation type lacks the built-in structure, and such a laser diodeis especially susceptive to mode leakage. Thereby, there appears a largedifference in the oscillation gain in the direction in which the lowrefractive index cladding layer is provided and in the direction inwhich no such low refractive cladding regions are provided. With this,it becomes possible to attain a large polarization ratio with the laserdiode of such a construction.

Thus, according to the eleventh mode of the present invention, itbecomes possible to control the polarization direction of the laser beamproduced with the surface-emission laser diode in a specific directionwhile maintaining the single fundamental transverse mode laseroscillation for the laser diode up to high output power.

(Twelfth Mode of Invention)

According to the twelfth mode of the present invention, there isprovided a surface-emission laser diode as set forth in any of the firstthrough eleventh mode of the invention, wherein the region of relativelylow refractive index, provided in spatial overlapping with the lasercavity region, has an anisotropic shape.

By forming the low refractive index core region of Al oxide formed inspatial overlapping with the laser oscillation region to have ananisotropic form such as rectangular form or elliptical form, thereappears anisotropy in the lateral optical confinement similarly to thecase of the tenth mode of the present invention, and it becomes possibleto change the transverse mode distribution between the anisotropicdirections. Thereby, it becomes possible to cause selective laseroscillation of the fundamental transverse mode with polarization in aspecific direction, by adjusting the width and effective refractiveindex of the low refractive index core appropriately such that thereappears a large coupling between the mode and the gain region in aspecific direction.

Here, it should be noted that the higher-order transverse mode ischaracterized by small electric amplitude in the laser oscillationregion, and because of this, higher-order transverse mode is susceptibleto the influence of the antiguiding structure. Because of the inherentlysmall coupling with the gain region, it becomes possible with such asurface-emission laser diode to suppress the higher-order transversemode laser oscillation effectively by providing such an antiguidingstructure.

Thus, with the twelfth mode of the invention, a surface-emission laserdiode is obtained such that the polarization direction is controlled ina specific direction and the single fundamental transverse modeoperation is maintained up to high output state.

(Thirteenth Mode of Invention)

According to the thirteenth mode of the present invention, there isprovided a surface-emission laser diode according to any of the firstthrough twelfth mode of the invention, wherein the active layer isformed of III-V elements and wherein the group III element constitutingthe active layer includes any or all of Ga and In, the group V elementconstituting the active layer includes any or all of As, N, Sb and P.

According to the thirteenth mode of the invention, it becomes possibleto obtain a device having excellent temperature characteristics andsuitable for the optical source of an optical communication system.

(Fourteenth Mode of Invention)

According to a fourteenth mode of the present invention, there isprovided a surface-emission laser array, in which a number of thesurface-emission laser diodes of any of the first through thirteenthaspect of the present invention are arranged to form an array.

The surface-emission laser array of the fourteenth mode of the presentinvention is capable of oscillating with the single fundamentaltransverse mode up to high output power.

(Fifteenth Mode of Invention)

According to the fifteenth mode of the present invention, there isprovided an optical interconnection system that uses any of thesurface-emission laser diode of any of the first through thirteenthmodes of the present invention or the surface-emission laser array ofthe fourteenth mode of the present invention as an optical source.

With the use of surface emission laser diode of the first throughthirteenth modes of the present invention or with the user of thesurface-emission laser array of the fourteenth mode of the presentinvention, a laser oscillation of single fundamental transverse mode isachieved up to high output power state. Thereby, a large opticalcoupling is achieved with regard to an optical fiber. Further, in viewof the excellent stability of transverse mode with change of operationalstate of the device, it becomes possible to provide a highly reliableoptical interconnection system by using any of the foregoing as theoptical source.

(Sixteenth Mode of Invention)

According to the sixteenth mode of the present invention, there isprovided an optical communication system that uses the surface-emissionlaser diode of any of the thirst through thirteenth modes of the presentinvention or the surface-emission laser array according to a fourteenthmode of the present invention as the communication optical source.

With the surface-emission laser diode of the first through thirteenthmodes of the present invention or with the surface-emission laser arrayaccording to the fourteenth mode of the present invention, singlefundamental laser oscillation is obtained up to high output state, andlarge coupling coefficient is attained with regard to an optical fiber.Further, because of the stabilized transverse mode with regard to thechange of operational state of the laser diode, it becomes possible toprovide a highly reliable optical communication system by using such asurface-emission laser diode or surface-emission laser array for thecommunication optical source. Further, because of the large output powerin the fundamental transverse mode, the communication system that usesthe laser diode or laser array of the present invention is capable ofperforming long-distance communication.

(Seventeenth Mode of Invention)

According to the seventeenth mode of the present invention, there isprovided an electro-photographic system that uses any of thesurface-emission laser diode of any of the first through thirteenthaspects of the present invention and the surface-emission laser arrayaccording to the fourteenth aspects of the present invention, as thewriting optical source.

The surface-emission laser diode of the first through thirteenth modesor the surface-emission laser array according to the fourteenth mode ofthe present invention, it becomes possible to maintain the singlefundamental transverse mode laser oscillation up to high output power.Further, an output beam having a circular cross section is obtained. Inaddition, with the case of using the laser diodes in the form of array,a high positional relationship is guaranteed between the output laserbeams. Thereby, it is possible to focus a large number of laser beamswith high reproducibility while using a single lens.

Thereby, the optical system is simplified and the electro-photographicsystem can be constructed with low cost. Further, because large outputpower is obtained in the fundamental transverse mode, it becomespossible to write images with high-speed, particularly by using a laserdiode array. Thereby, it becomes possible to realize a high-speedelectro-photographic system.

(Eighteenth Mode of Invention)

According to the eighteenth mode of the present invention, there isprovided an optical disk system that uses the surface-emission laserdiode of any of the first through thirteenth modes of the presentinvention or the surface-emission laser array of the fourteenth mode ofthe present invention as the read/out optical source.

With the use of the surface-emission laser diode of any of the firstthrough thirteenth modes of the present invention or thesurface-emission laser array of the fourteenth mode, it becomes possibleto obtain high output power while maintaining the single fundamentalmode laser oscillation. Further, it becomes possible to obtain an outputlaser beam having a circular cross section. Thereby, it becomes possibleto construct a highly reliable optical disk system. Particularly, byusing the laser diode in the form of array, it becomes possible toconstruct a high-speed optical disk system capable of reading and/orwriting with high density.

EXAMPLE 1

FIG. 1 shows a surface-emission laser diode according to Example 1 ofthe present invention, wherein the laser diode of FIG. 1 is asurface-emission laser diode having an active layer formed of a multiplequantum well structure of GaAs/Al_(0.15)Ga_(0.85)As and operating in the0.85 μm band.

Hereinafter, the fabrication process of the surface-emission laser diodewill be explained according to the fabrication process thereof.

Referring to FIG. 1, the surface-emission laser diode is based on thelayered structure formed by an organic metal chemical vapor deposition(MOCVD) process, wherein the layered structure is formed by usingtrimethyl aluminum (TMA), trimethyl gallium (TMG) and trimethyl indium(TMI) for the source of the group III elements and by using an arsine(AsH₃) gas for the group V source. Further, CBr₄ is used for the p-typedopant and H₂Se is used for the n-type dopant.

More specifically, the device of FIG. 1 is constructed on an n-GaAssubstrate 101 and includes an n-GaAs buffer layer 102 formed on thesubstrate 101, and an n-Al_(0.9)Ga_(0.1)As/Al0_(.15)Ga_(0.85)As lowersemiconductor distributed Bragg reflector 103 is formed on the bufferlayer 102, wherein the lower distributed Bragg reflector 103 includestherein repetition of an n-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As pairrepeated for 36 times.

Further, a cavity spacer layer 104 of undoped Al_(0.15)Ga_(0.85)As isformed on the lower distributed Bragg reflector 103, and a multiplequantum well active layer 105 of GaAs/Al_(0.15)Ga_(0.85)As is formedfurther on the cavity spacer layer 104.

Further, another cavity spacer layer 106 of undoped Al_(0.15)Ga_(0.85)Asis formed on the active layer 105, and ap-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As upper semiconductordistributed Bragg reflector 107 is formed on the cavity spacer layer106, wherein the upper semiconductor distributed Bragg reflector 107includes therein repetition of ap-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As pair repeated for 20 times.Further, the uppermost Al_(0.15)Ga_(0.85)As layer of the upperdistributed Bragg reflector 107 is doped with a p-type dopant (carbon)with high concentration level at the surface part thereof to form acontact layer (not shown).

Further, there is provided a p-type AlAs selectively oxidizing layer 108inside the P-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As layer for currentconfinement. Here, the p-AlAs selectively oxidizing layer 108 includesan oxidized region (shown in black; the same designation is used also inother Examples) formed by selective oxidation from the etching edgesurface in a high temperature ambient containing water vapor and anon-oxidized region.

It should be noted that the surface-emission laser diode includes asquare-shaped mesa structure formed by the steps of: forming, after thecrystal growth process of the layered structure, a square resist patternhaving a size of 30 μm for each edge, on the layered structure formed bythe crystal growth process by a known photolithographic process, andremoving the layered structure starting from the top surface of theP-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As upper distributed Braggreflector to a mid position of then-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As lower distributed Braggreflector 103.

Next, there is formed a resist opening of a square shape in alignmentwith the central part of the mesa structure, and ion implantation ofoxygen molecules is made into the central part of the mesa structurewhile using the resist pattern as a mask. Thereby, the oxygen ions areinjected into an oxygen ion implantation region 113.

Next, the resist is removed, and a selective oxidation is made for thep-AlAs selective oxidizing layer 108 in a high temperature ambientcontaining water vapor in the direction parallel to the substrate,starting from the etching edge surface to the central part of the mesastructure. Thereby, there is formed a current confinement structure as aresult of such a selective oxidation process.

At the same time to the selective oxidation processing conducted in thehigh temperature ambient containing water vapor, there is achieved anannealing process in the oxygen ion implantation region 113, andoxidation of the Al_(0.9)Ga_(0.1)As mixed crystal proceedssimultaneously in the region 113. Thereby, there is formed a selectiveoxidation region of relatively low refractive index at the central partof the mesa structure in correspondence to the region 113, wherein theselective oxidation region has a refractive index lower than therefractive index of the surrounding region. In the illustrated example,the region 113 of oxygen ion implantation has an edge length of 15 μm,while the current confinement region has an edge length of 10 μm.

Next, an SiO₂ layer 109 is formed on the entire wafer surface by a CVDprocess. After formation of the SiO₂ layer 109, there is conducted aspin coating process of an insulating resin layer 110, and theinsulating resin layer is removed from the mesa region.

Next, the SiO₂ layer 109 is removed in correspondence to the part wherethe insulation resin layer has been removed, and a square resist patternis formed in correspondence to an optical beam exit region to be formedon the mesa structure, with an edge length of 10 μm. Further, a p-typeelectrode material is deposited by an evaporation deposition process,and the electrode material thus deposited is removed in correspondenceto the foregoing beam exit part by a liftoff process. With this, ap-side electrode 111 is formed.

Next, the rear surface of the n-GaAs substrate 101 is polished, and ann-side electrode 112 is formed on the rear surface of the substrate byan evaporation deposition process. Further, ohmic contact is achievedfor each of the electrodes 111 and 112 by conducting an annealingprocess.

FIG. 2 is a diagram showing a part of the region 113 in which the oxygenion implantation has been conducted in FIG. 1.

It should be noted that the Al_(0.9)Ga_(0.1)As mixed crystal located inthe region where the oxygen ion implantation has been made undergoesoxidation with the thermal annealing process conducted after the oxygenion implantation process by causing reaction with the Al atoms, and as aresult, there is formed a region of relatively low refractive index(shown in black) wherein such a region has a relatively low refractiveindex as compared with the surrounding mixed crystal ofAl_(0.9)Ga_(0.1)As. Thereby, there is formed an antiguiding structure inthe cavity structure. Here, it should be noted that an AlGaAs mixedcrystal shows an oxidation rate that increases with the Al content. ONthe other hand, there occurs little oxidation in the GaAs layer in thepresent device.

It should be noted that, with regard to the oxidation structureconstituting the antiguiding structure at the mesa central part, allwhat is required is that it has a relatively low refractive index ascompared with the surrounding region. Thus, it is not necessary that theAlGaAs mixed crystal is completely oxidized. In other words, it ispossible with the device of the present example to adjust the dose ofoxygen ion implantation such that there is obtained a refractive indexprofile in conformity with the design.

Here, it should be noted that the AlGaAs mixed crystal, which issubjected to the oxygen ion implantation after formation thereof, isformed to have a thickness in anticipation of the refractive index andfilm thickness after the ion implantation and thermal annealing processalready at the time of the crystal growth process, such that the phasecondition of multiple reflection of the distributed Bragg reflector ismet.

Thus, the region where there occurs a decrease of the refractive indexafter the ion implantation and thermal annealing process, constitutesthe low refractive index layer of the distributed Bragg reflector afterthe oxygen ion implantation and thermal annealing process. Thus, itshould be noted that the p-Al_(0.9)Ga_(0.1)As layer to which the ionimplantation is to be made has a thickness different from thep-Al_(0.9)Ga_(0.1)As layers above and below. More specifically, such alayer is formed to have an increased thickness as compared with otherp-Al_(0.9)Ga_(0.1)As layers in view of the change of the refractiveindex to be caused therein. Including the oxygen ion implantationregion, the thickness of the layers constituting the distributed Braggreflector is chosen so as to satisfy the phase condition of multiplereflection of the Bragg reflector. In other words, the thickness of thelayers constituting the distributed Bragg reflector is chosen such thatthere occurs a phase change of π/2 in each of the layers for the laseroscillation beam.

By setting the thickness of the AlGaAs mixed crystal in the ionimplantation region such that the Bragg condition is satisfied with therefractive index after the oxidation processing, there is achieved ahigh reflectance at the mesa central part (where the oxygen ionimplantation has been made) at the resonant wavelength, while thereoccurs a decrease of reflectance in the region offset from the mesacentral part near the mesa peripheral part where no oxygen ionimplantation has been made, because of offset from the phase condition.Because the higher-order transverse mode has large mode amplitude insuch a region where the reflectivity is decreased, there occurs anincrease of mirror loss with such a region, and the effect ofsuppressing the laser oscillation in higher-order transverse mode iseven more enhanced.

With the surface-emission laser diode of FIG. 1, the higher-ordertransverse mode is displaced toward the peripheral part of the mesastructure due to the existence of the antiguiding structure of lowrefractive index region formed by the oxygen ion implantation into theAlGaAs mixed crystal and subsequent annealing process as represented inFIG. 3B, and overlapping of the higher-order transverse modedistribution with the gain region, which is determined by the currentinjection diameter and shown in FIG. 3B schematically, is reduced.Further, in view of the decrease of reflectivity of the distributedBragg reflector at the peripheral part of the mesa structure, the laseroscillation with the higher-order transverse mode is effectivelysuppressed. The laser diode could maintain the single fundamentaltransverse mode up to higher output power as compared with aconventional device.

EXAMPLE 2

FIGS. 4 and 5 are diagrams for explaining the surface-emission laserdiode according to Example 2 of the present invention.

Referring to FIG. 4, the laser diode of Example 2 is formed similarly tothe laser diode of FIG. 1 up to an intermediate part of the p-typedistributed Bragg reflector by a crystal growth process.

With Example 2, the crystal growth is made up to the foregoingintermediate part of the p-type distributed Bragg reflector, wherein thecrystal growth is interrupted in this state and ion implantation of themolecules containing oxygen is conducted to the region that forms thecentral part of the mesa structure. After the ion implantation process,thermal annealing process is conducted for recovering the crystaldefects caused by ion implantation, and the remaining part of the deviceis formed by conducting a regrowth process.

As a result of the thermal annealing process conducted for recoveringthe crystal defect and as a result of the heating at the time of theregrowth process of forming the remaining part of the device, the region113 where the oxygen ion implantation has been made is selectivelyoxidized, and an antiguiding structure characterized by a low refractiveindex as compared with the surrounding region is formed incorrespondence to the region 113.

As a result of such a fabrication process, it becomes possible to make ashallow oxygen ion implantation, and it becomes possible to achieve avery steep ion implantation profile with excellent controllability.Thereby, it becomes possible to confine the region of modifiedrefractive index formed as a result of the thermal annealing processwith high precision in the growth direction, and the adjustment of phasecondition of multiple reflection in the distributed Bragg reflector isfacilitated substantially.

Further, as shown in FIG. 5, it is possible to adjust the phasecondition more precisely by providing a distributed Bragg reflector ofGa_(0.5)In_(0.5)P/Al_(0.15)Ga_(0.85)As before and after the distributedBragg reflector of Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As.

More specifically, FIG. 5 shows the example in which a distributed Braggreflector of Ga_(0.5)In_(0.5)P/Al_(0.15)Ga_(0.85)As is provided adjacentto the distributed Bragg reflector formed of one pairAl_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As.

Here, it should be noted that Ga_(0.5)In_(0.5)P can be grown in latticematching with a GaAs substrate and that it does not contain Al as aconstituent element. Thus, there occurs little oxidation in theGa_(0.5)In_(0.5)P layer even in the case there has been conducted oxygenion implantation.

Thus, by using such a Ga_(0.5)In_(0.5)P layer, it becomes possible tocarry out oxidation of a specific AlGaAs layer even in the case thereexists a range in the depth distribution of the injected oxygen atoms.Further, it becomes possible to adjust the reflection wavelength of thedistributed Bragg reflector at the mesa central part (reflectionwavelength for the fundamental transverse mode) more precisely and withexcellent controllability, by selectively oxidizing a specific AlGaAslayer.

Further, by interrupting the growth of the upper distributed Braggreflector with Example 2 as shown in FIG. 4 and conducting the oxygenion implantation in advance to the growth of the remaining part of thedistributed Bragg reflector, it becomes possible to obtain a steep ionimplantation profile for the oxygen atoms. Further, by providing theGa_(0.5)In_(0.5)P/Al_(0.15)Ga_(0.85)As distributed Bragg reflector asshown in FIG. 5, the region of selective oxidation by ion implantationprocess is controlled more precisely. Because of the steep ionimplantation profile, the number of layers of theGa_(0.5)In_(0.5)P/Al_(0.15)Ga_(0.85)As distributed Bragg reflectorprovided for preventing unwanted oxidation can be reduced, and theadversary effect on the reflectivity of the distributed Bragg reflectorcan be held minimum.

With the surface-emission laser diode of Example 2 thus fabricated, thedistribution of higher-order transverse mode is shifted to theperipheral part of the mesa structure as shown in FIG. 3B as a result ofexistence of the antiguiding structure formed by selective oxidationconducted by the oxygen ion implantation into the AlGaAs mixed crystaland subsequent thermal annealing process, similarly to the device ofExample 1.

Thereby, overlapping of the higher-order transverse mode with the gainregion, represented in FIG. 3B schematically and determined by thecurrent injection diameter, is reduced. Further, because of the decreaseof the reflectivity of the distributed Bragg reflector at the peripheralpart of the mesa structure, laser oscillation with the higher-ordertransverse mode is effectively suppressed and it becomes possible tomaintain the single fundamental transverse mode laser oscillation up tothe state of high output power. Further, because it is possible tocontrol the reflection condition of the distributed Bragg reflector withexcellent controllability, it becomes possible to obtain the laser diodewith excellent characteristics such as the oscillation thresholdcharacteristics.

EXAMPLE 3

FIG. 6 explains the surface-emission laser diode of Example 3.

Referring to FIG. 6, it will be noted that the construction of theregion surrounding the antiguiding structure 113 in the surface-emissionlaser diode of FIG. 1 is changed in the case of the device of Example 3.

In more detail, in the example of FIG. 6, the Al_(0.9)Ga_(0.1)As mixedcrystal layer provided for forming the low refractive index layer afterthe selective oxidation process conducted by ion implantation of themolecules containing oxygen and subsequent thermal annealing process, isgrown with increased doping concentration level as compared with theAl_(0.9)Ga_(0.1)As layers used for other regions. The process of devicefabrication itself is identical with the case of the device of Example1.

It should be noted that a semiconductor material doped to highconcentration level shows distinctive optical absorption caused by freecarrier absorption. In the case of a p-type semiconductor, inparticular, there appears intra-valence band absorption in addition tothe foregoing free carrier absorption, and because of this, such asemiconductor material shows the nature of increased optical absorptionfor the long-wavelength band optical radiation.

Thus, with the device of Example 3, in which the Al_(0.9)Ga_(0.1)Aslayer that constitutes the low refractive index region after beingapplied with the selective oxidation process conducted by the ionimplantation process and subsequent thermal annealing process, is formedwith high concentration doping at the time of the crystal growth processthereof, it becomes possible to decrease the serial resistance of thep-type distributed Bragg reflector and further increase the absorptionloss at the antiguiding structure part due to the free carrierabsorption and intra-valence band absorption.

On the other hand, in the part of such a p-type Al_(0.9)Ga_(0.1)As layerwhere the oxygen ion implantation has been made, there occurs formationof insulation material of AlOx, which is transparent to the oscillationwavelength.

Thus, the absorption loss is vanished with regard to the fundamentaltransverse mode having a large mode amplitude at the central part of themesa structure (ion injection region), and there appears absorption onlyto the higher-order transverse mode having a large mode amplitude in theregion offset from the mesa central region and close to the periphery ofthe mesa structure. Thus, with Example 3, it becomes possible tosuppress the laser oscillation at higher-order transverse mode as aresult of such increase of the absorption loss.

Further, with the laser diode of Example 3, it is possible to enhancethe absorption of the higher-order transverse mode by using theconstruction of FIG. 7.

FIG. 7 shows another example of the oxygen ion injection region 113 ofthe surface-emission laser diode of Example 1.

Referring to FIG. 7, there is provided an Al_(0.9)Ga_(0.1)As layer dopedto the p-type with high concentration level similarly with the exampleof FIG. 6, except that there is provided an Al_(0.9)Ga_(0.1)As layerwith the thickness of ¾λ.

In a standing wave formed in a distributed Bragg reflector, the node andanti-node of electric field are repeated alternately in the thicknessdirection with separation of λ/4 (each π/2 in terms of phase). In an nλcavity, the interface crossing from an Al_(0.9)Ga_(0.1)As layer to anAl_(0.15)Ga_(0.85)As as viewed from the cavity becomes a node of theelectric field. Further, the interface crossing fromAl_(0.15)Ga_(0.85)As to Al_(0.9)Ga_(0.1)As as viewed from the cavityforms an anti-node of the electric field.

Thus, with the construction of FIG. 7 that includes theAl_(0.9)Ga_(0.1)As layer with the thickness of ¾λ, there inevitablyoccurs a situation that the Al_(0.9)Ga_(0.1)As includes the anti-node ofthe standing wave. In FIG. 7, it should be noted that the anti-node ofthe standing wave is formed at a location indicated by an arrow.

In such a location corresponding to the anti-node of the standing wave,the electric field strength of the optical radiation takes a largevalue, while there occurs strong absorption at this location because ofthe use of the Al_(0.9)Ga_(0.1)As layer doped with high concentrationlevel. Thus, it becomes possible to increase the absorption loss of thehigher-order transverse mode further as explained above, and the laseroscillation in higher-order transverse mode is suppressed moreeffectively.

By doping the AlGaAs layer that constitutes the antiguiding structureafter conversion to low refractive index region by conducting selectiveoxidation including the steps of ion implantation of the moleculescontaining oxygen and subsequent thermal annealing process, such thatthe AlGaAs layer has high doping concentration as compared with otherAlGaAs layers at the time of the crystal growth thereof, it becomespossible to form an absorption region of the higher-order transverseoptical radiation around the low refractive index region. Thus, with thelaser diode of Example 3, it becomes possible to maintain the singlefundamental transverse mode laser oscillation up to high output powerstate.

EXAMPLE 4

FIG. 8 is a diagram showing the surface-emission laser diode of Example4, wherein the surface-emission laser diode of FIG. 8 is a device thatuses a GaInNAs/GaAs multiple quantum well structure for the active layerand operates in the 1.3 μm band.

Hereinafter, the laser diode of FIG. 8 will be explained according tothe fabrication process thereof.

Referring to FIG. 8, the surface-emission laser diode includes asemiconductor layered structure formed of an MOCVD process similarly tothe device of Example 1, wherein the growth of the layered structure isconducted by using trimethyl aluminum (TMA), trimethyl gallium (TMG) andtrimethyl indium (TMI) for the source of the group III element andarsine (AsH₃) gas for the source of the group V element. Further, CBr₄is used for the p-type dopant while H2Se is used for the n-type dopant.Further, dimethyl hydrazine (DMHy) is used for the nitrogen source ofthe active layer.

More in detail, the device of FIG. 8 is formed on an n-GaAs substrate201 and includes an n-GaAs buffer layer 202 formed on the n-GaAssubstrate 201, wherein a lower semiconductor distributed Bragg reflector203 including therein 36 repetitions of an n-Al_(0.9)Ga_(0.1)As/GaAspair is formed on the n-GaAs buffer layer 202.

Further, a cavity spacer layer 204 of non-doped GaAs, a multiple quantumwell active layer 205 of GaInNAs/GaAs structure, and a cavity spacerlayer of undoped GaAs are formed on the lower semiconductor distributedBragg reflector 203 consecutively, and an upper semiconductordistributed Bragg reflector 207 of p-Al_(0.9)Ga_(0.1)As/GaAs structureincluding 20 repetitions of the p-Al_(0.9)Ga_(0.1)As/GaAs pair is formedon the undoped GaAs cavity spacer layer 206. Further, there is formed acontact layer (not shown) in the GaAs layer at the uppermost layer ofthe upper distributed Bragg reflector 207 by increasing the dopingconcentration of the p-type dopant (carbon) at the surface part of thecontact layer. Further, there is provided a p-AlAs selectively oxidizinglayer 208 inside the foregoing p-Al_(0.9)Ga_(0.1)As/GaAs uppersemiconductor distributed Bragg reflector.

With the device of FIG. 8, it should be noted that the growth of then-type distributed Bragg reflector 203 is interrupted during the crystalgrowth process thereof, wherein there is conducted an oxygen ionimplantation process to the mesa central part similarly to the device ofExample 2. Further, a recovering annealing process is conducted, andcrystal growth of the remaining part of the device structure isconducted thereafter.

At the time of the thermal annealing process for recovering the crystaldefects and the crystal growth process for growing the remaining part ofthe device structure, the region 213 injected with oxygen undergoesselective oxidation, and as a result, there is formed an antiguidingstructure of low refractive index in correspondence to the region 213.

Thereafter, the processes such as mesa formation, selective oxidation ofthe p-AlAs selective oxidizing layer 208, which is conducted in the hightemperature water vapor ambient, planarization by filling the trenchesat both sides of the mesa structure by a resin, and formation ofelectrodes are conducted similarly to the process of Example 1, and withthis the surface-emission laser diode of FIG. 8 is obtained.

It will be noted that the surface-emission laser diode of Example 4 isdistinct over the device of Examples 1-3 in the point that theantiguiding structure 213 of low refractive index layer formed by theion implantation process and subsequent thermal annealing process, isnow provided inside the n-type distributed Bragg reflector. Here, thelayers surrounding the antiguiding structure can be formed similarly toany of Examples 1-3, except that the conductivity type thereof ischanged from p-type to n-type.

By forming the antiguiding structure inside the n-type distributed Braggreflector as in the case of the device of Example 4, it becomes possibleto suppress the higher-order transverse mode efficiently similarly toExamples 1-3, and at the same time, it becomes possible to reduce thedevice resistance significantly.

In the case an antiguiding structure is provided in a p-typesemiconductor layer so as to overlap spatially with the currentinjection region, the holes avoid the antiguiding structure as it flowsthrough the device structure and are injected into the active layerafter being confined by the current confinement structure to the centralpart of the mesa structure. Thereby, there arises a problem that thecurrent path of the holes is elongated and that the current path isnarrowed. Further, because the carrier mobility is smaller in a p-typesemiconductor material as in an n-type semiconductor material by theorder of one, this also contributes to the increase of deviceresistance.

In the case of Example 4 in which the antiguiding structure is formed inthe n-type distributed Bragg reflector, there occurs little increase ofresistance in view of the fact that there is provided no currentconfinement structure between the antiguiding structure and the activelayer and in view of the inherently large mobility of electrons, even inthe case there exists an insulation or high resistance region at thecentral part of the mesa structure. Thus, it becomes possible tosuppress the device resistance substantially equal to the case ofconventional devices having no antiguiding structure. Because theantiguiding structure performs similarly against the transverse mode inany of the cases in which it is provided in the p-type distributed Braggreflector and in which it is provided in the n-type lo distributed Braggreflector, and thus, it is possible to obtain an effect similar toExamples 1 and 2 in the device of Example 4.

With the device of Example 4, it is possible to operate in singlefundamental mode up to high output state similarly to the device ofExamples 1-3, while the device of Example 4 can reduce the deviceresistance further. With reduced device resistance, it becomes possibleto achieve operation with higher output power.

EXAMPLE 5

FIG. 9 shows the surface-emission laser diode according to Example 5 ofthe present invention, wherein the device of FIG. 9 is asurface-emission laser diode having a GaInNAs/GaAs multiple quantum wellstructure for the active layer and operable in the 1.3 μm band.

Hereinafter, the structure thereof will be explained together with thefabrication process.

Referring to FIG. 9, the surface-emission laser diode uses a layeredstructure formed similarly to the device of Example 4.

More specifically, the device of FIG. 9 is constructed on an n-GaAssubstrate 301 carrying thereon an n-GaAs buffer layer 302, wherein thereis provided a lower semiconductor Bragg reflector 303 including therein36 repetitions of an n-Al_(0.9)Ga_(0.1)As/GaAs pair.

On the lower semiconductor distributed Bragg reflector 302, there isprovided a non-doped GaAs cavity spacer layer 304, and an active layer305 of GaInNAs/GaAs multiple quantum well structure is formed on thecavity spacer layer 304.

Further, an undoped GaAs cavity spacer layer 306 is formed on the activelayer 305, and an upper distributed Bragg reflector 307 includingtherein 20 repetitions of a p-Al_(0.9)Ga_(0.1)As/GaAs pair is formed onthe upper cavity layer 306. Further, a contact layer (not shown) isformed by the uppermost GaAs layer of the upper distributed Braggreflector 307, wherein the contact layer is doped with a p-type dopant(carbon) with high concentration level at the surface part thereof.

After formation of the layered structure by the foregoing crystal growthprocess, there is formed a square resist opening having an edge lengthof 3 μm by a known photolithographic process with the surface-emissionlaser diode of FIG. 9, and oxygen ion implantation is conducted theretofrom the surface of the p-Al_(0.9)Ga_(0.1)As/GaAs upper semiconductordistributed Bragg reflector 307. After thermal annealing process causingselective oxidation in the oxygen ion injection part, there is formed anantiguiding structure formed of a low refractive index region 313 in thep-type distributed Bragg reflector 307.

FIG. 9 also shows the effective refractive index profile in thedirection parallel to the substrate surface. As can be seen in FIG. 9,the part provided with the low refractive index region has a relativelysmaller effective refractive index.

Thereafter, a resist pattern having a size of 10 μm is formed inalignment with region where the oxygen ion implantation has been made,and hydrogen ion implantation is conducted into the laser structurewhile using the resist pattern as a mask, such that the hydrogen ionsare distributed with a peak depth deeper than the peak depth of theoxygen ions. With this, the resistance of the device structure isincreased in correspondence to the part where the hydrogen ionimplantation has been made. Here, it should be noted that the desiredhigh resistance region can be formed also by conducting oxygen ionimplantation process in place of the hydrogen ion implantation process.

Next, a p-side electrode 311 is formed on the device surface and ann-side electrode 312 is formed on the rear surface of the substrate 301after polishing such a rear surface. Further, there are formed ohmiccontact with such p-side electrode 311 and the n-side electrode 312 byconducting a thermal annealing process.

Here, it should be noted that there occurs little refractive indexchange at the part where the hydrogen ion implantation has been madewith such a surface-emission laser diode that uses a current confinementstructure formed by the hydrogen ion implantation process. This meansthat such a laser diode lacks a built-in optical confinement structure(guiding structure), and because of this, the effect of the antiguidingstructure over the mode distribution is increased as compared with thesurface-emission laser diode that uses oxidation confinement.

Thus, with the laser diode of Example 5, it becomes possible to displacethe higher-order transverse mode to the region outside the gain regionwith excellent efficiency, and laser oscillation with higher-ordertransverse mode can be suppressed with further high efficiency.

In the device of Example 5, it should be noted that the region of oxygenion implantation is provided close to the device surface for avoidingexcessive effect to the fundamental transverse mode. Further, the widthof the oxygen ion implantation region is set to about 3 μm for the samepurpose.

Further, in order to increase the resistance of the p-type Braggreflector 307, the laser diode of Example 5 avoids overlapping of theoxygen ion injection region and the hydrogen ion injection region byincreasing the acceleration voltage at the time of the hydrogen ionimplantation process. With the device of Example 5, it becomes possibleto obtain a single fundamental transverse mode operation up to very highoutput state.

FIG. 10 shows an example in which the antiguiding structure 313 isformed inside the n-type distributed Bragg reflector similarly to thedevice of Example 4.

Thus, in the example of FIG. 10, the crystal growth process for formingthe n-type distributed Bragg reflector 303 is interrupted and the region313 of oxygen ion implantation is subjected to selective oxidation byconducting ion implantation of molecules containing oxygen, followed bythermal annealing process. Thereafter, the remaining part of device isformed by resuming the crystal growth process.

With the device of FIG. 10, in which the antiguiding structure isprovided inside the n-type distributed Bragg reflector, it becomespossible to reduce the device resistance similarly to the device ofExample 4, in addition to the very large effect of suppressing thehigher-order transverse mode laser oscillation similarly to the deviceof FIG. 9.

In fact, the device of Example 5 can perform the single fundamentaltransverse mode operation up to very high output state, whilemaintaining sufficiently low device resistance. Because of the reduceddevice resistance, the surface-emission laser diode 5 of Example 5 canprovide further higher output operation.

EXAMPLE 6

FIGS. 11A and 11B are diagrams showing the surface-emission laser diodeof Example 6, wherein it should be noted that FIG. 11B shows a plan viewof the laser diode of FIG. 11A. The surface-emission laser diode ofFIGS. 11A and 11B is a laser diode having an active layer ofGaInNAs/GaAs multiple quantum well structure and operating in the 1.3 μmwavelength band.

Hereinafter, the device structure will be explained together with thefabrication process thereof.

The surface-emission laser diode of FIGS. 11A and 11B is formed byconducting a crystal growth process similar to the one used with Example4.

More specifically, the device of FIGS. 11A and 11B is constructed on ann-GaAs substrate 301 carrying thereon a n-GaAs buffer layer 302, whereinthe n-GaAs buffer layer 302 carries thereon a lower semiconductordistributed Bragg reflector 303 including therein 36 repetitions of ann-Al_(0.9)Ga_(0.1)As/GaAs pair. Further, an undoped GaAs cavity spacerlayer 304 is formed on the lower semiconductor distributed Braggreflector 303, and an active layer 305 of GaInNAs/GaAs multiple quantumwell structure is formed on the GaAs cavity spacer layer 304.

On the active layer 305, there is provided another cavity spacer layer306 of undoped GaAs, and an upper distributed Bragg reflector 307including therein 20 repetitions of a p-Al_(0.9)Ga_(0.1)As/GaAs pair isformed further on the cavity spacer layer 306.

Further, there is provided a contact layer (not shown) in correspondenceto the GaAs layer at the uppermost part of the upper distributed Braggreflector 307 such that there is provided a high concentration doping ofp-type dopant (carbon) at the surface part of such a GaAs layer.

After the crystal growth process, there is formed a square resistpattern having a resist opening in correspondence to the low refractiveindex regions 1 and 2 of FIG. 11B by using a known photolithographicprocess, and oxygen ion implantation is conducted through the surface ofthe P-Al_(0.9)Ga_(0.1)As/GaAs upper semiconductor distributed Braggreflector while using the resist pattern as a mask.

Next, a thermal annealing process is conducted and there is achieved aselective oxidation process in correspondence to where the ionimplantation of oxygen has been made. Thereby, the low refractive indexregion 1 serving for the antiguiding structure is formed at the centralpart of the device and the low refractive index region 2 (claddingregion) is formed at the peripheral part of the device for confining thetransverse mode.

Here, it should be noted that the opening d1 of the resist patterncorresponding to the low refractive index region 1 is set to have a sizeof 5 μm, while the opening d2 of the resist pattern corresponding to thelow refractive index region 2 is set to have a size of 30 μm.

FIG. 11A also shows the effective refractive index profile in thedirection parallel to the substrate surface.

Referring to FIG. 11A, it can be seen that the effective refractiveindex is reduced relatively in correspondence to the part where the lowrefractive index region is provided. Further, Further, it can be seenthat there is formed a high refractive index region at both sides of thelow refractive index core at the central part of the device. Further, itcan be seen that there is formed a cladding region of low refractiveindex region at further outer side of the high refractive index region.

After this, there is formed a square resist pattern in alignment withthe oxygen ion implantation part, and ion implantation of hydrogen isconducted into the device structure while using the resist pattern as amask, such that the hydrogen distribution peak is located at a leveldeeper than the oxygen distribution peak in the oxygen distributionpart. With this, the resistance of the layered structure is increased incorrespondence to the hydrogen ion injection part. In the presentexample, the resist pattern is formed to have the edge size d3 of 8 μm.

It should be noted that the foregoing high resistance region can beformed also by ion implantation of oxygen.

Next, a p-side electrode 311 is formed on the device surface and ann-side electrode 312 is formed on the rear surface of the substrate 301after polishing of the rear surface. Further, by conducting an annealingprocess, the p-side electrode 311 and the n-side electrode 312 form anohmic contact.

With the surface-emission laser diode that uses the current confinementstructure formed by hydrogen ion implantation process, there is littlerefractive index change in the part where the hydrogen ion implantationhas been made, while this means that such a laser diode does not have abuilt-in optical confinement structure (waveguide structure). In such alaser diode, the antiguiding structure provides a larger effect ascompared with the case of forming such an antiguiding structure in asurface-emission laser diode having oxidized current confinementstructure.

Thus, the use of the antiguiding structure in such a surface-emissionlayer diode of hydrogen ion implantation type enhances the effect ofdisplacing the higher-order mode distribution to the peripheral part ofthe device structure.

On the other hand, there is a tendency with such a structure that thereoccurs extensive lateral mode leakage, while such lateral mode leakageincreases the optical loss. The surface-emission laser diode of thepresent example successfully eliminates the problem of optical losscaused by mode leakage, by providing the cladding structure at theperipheral part of the device in the form of outer low refractive indexregion.

Further, it should be noted that the effective refractive index in thelow refractive index regions 1 and 2 can be adjusted by adjusting thethickness and depth of the Al oxidation region.

It should be noted that the oxygen ion implantation into the lowrefractive index regions 1 and 2 can be conducted separately in twodifferent steps, by using different acceleration voltages and differention currents.

Further, it is possible to use different values for various sizes of thesurface-emission laser diode of the present embodiment. By choosing thesizes appropriately and by optimizing the ion implantation conditionsuch that there occur increase of coupling between the fundamentaltransverse mode and the gain region and such that coupling between thehigher-order transverse mode and the gain region is decreased, theeffect of the present invention can be increased further.

From the foregoing, the laser diode of Example 6 can perform the singlefundamental transverse mode operation up to higher output-states.

EXAMPLE 7

FIGS. 12A and 12B are diagrams showing the surface-emission laser diodeof Example 7, wherein it should be noted that FIG. 12B shows a plan viewof the laser diode of FIG. 12A. The surface-emission laser diode ofFIGS. 12A and 12B is a laser diode having an active layer of GaInAs/GaAsmultiple quantum well structure and operating in the 0.98 μm wavelengthband.

Hereinafter, the device structure will be explained together with thefabrication process thereof. The laser diode of FIGS. 12A and 12B aregrown with similar methods and similar means to the case of Example 4.

More specifically, the device of FIGS. 12A and 12B is constructed on ann-GaAs substrate 401 carrying thereon a n-GaAs buffer layer 402, whereinthe n-GaAs buffer layer 402 carries thereon a lower semiconductordistributed Bragg reflector 403 including therein 36 repetitions of ann-Al_(0.9)Ga_(0.1)As/GaAs pair. Further, an undoped GaAs cavity spacerlayer 404 is formed on the lower semiconductor distributed Braggreflector 403, and an active layer 405 of GaInAs/GaAs multiple quantumwell structure is formed on the GaAs cavity spacer layer 404.

On the active layer 405, there is provided another cavity spacer layer406 of undoped GaAs, and an upper distributed Bragg reflector 407including therein 20 repetitions of a p-Al_(0.9)Ga_(0.1)As/GaAs pair isformed further on the cavity spacer layer 406.

Further, there is provided a contact layer (not shown) in correspondenceto the GaAs layer at the uppermost part of the upper distributed Braggreflector 407 such that there is provided a high concentration doping ofp-type dopant (carbon) at the surface part of such a GaAs layer.

After the crystal growth process, there are formed a square resistpattern having a resist opening in correspondence to the low refractiveindex region 1 of FIG. 12B and a rectangular resist pattern having aresist opening in correspondence to the low refractive index region 2 ofFIG. 12B by using a known photolithographic process. Further, oxygen ionimplantation is conducted through the surface of theP-Al_(0.9)Ga_(0.1)As/GaAs upper semiconductor distributed Braggreflector while using the resist pattern as a mask.

Next, a thermal annealing process is conducted and there is achieved aselective oxidation process in correspondence to where the ionimplantation of oxygen has been made. Thereby, the low refractive indexregion 1 serving for the antiguiding structure is formed at the centralpart of the device and the low refractive index region 2 (claddingregion) is formed at the peripheral part of the device for confining thetransverse mode.

Here, it should be noted that the opening d1 of the resist patterncorresponding to the low refractive index region 1 is set to have a sizeof 5 μm, while the opening d2 of the resist pattern corresponding to thelow refractive index region 2 (cladding region) is set to have a size of40 μm in a first direction. The opening d2 is thereby formed to have asize d4 of 20 μm in the second direction.

FIG. 12A also shows the effective refractive index profile in thedirection parallel to the substrate surface.

Referring to FIG. 12A, it can be seen that the effective refractiveindex is reduced relatively in correspondence to the part where the lowrefractive index region is provided. Further, Further, it can be seenthat there is formed a high refractive index region at both sides of thelow refractive index core at the central part of the device. Further, itcan be seen that there is formed a cladding region of low refractiveindex region at further outer side of the high refractive index region.

After this, there is formed a square resist pattern in alignment withthe oxygen ion implantation part, and ion implantation of hydrogen isconducted into the device structure while using the resist pattern as amask, such that the hydrogen distribution peak is located at a leveldeeper than the oxygen distribution peak in the oxygen distributionpart. With this, the resistance of the layered structure is increased incorrespondence to the hydrogen ion injection part. In the presentexample, the resist pattern is formed to have the edge size d3 of 8 μm.

It should be noted that the foregoing high resistance region can beformed also by ion implantation of oxygen.

Next, a p-side electrode 311 is formed on the device surface and ann-side electrode 312 is formed on the rear surface of the substrate 301after polishing of the rear surface. Further, by conducting an annealingprocess, the p-side electrode 311 and the n-side electrode 312 form anohmic contact. With this, the surface-emission laser diode of FIGS. 12Aand 12B is obtained.

With the surface-emission laser diode of the present example, the regionthat includes the laser cavity region and sandwiched by the lowrefractive index regions 2 (cladding regions) in the directionperpendicular to the laser cavity direction is formed to have differentwidths in the first direction and second direction. With this, the modedistribution is changed in the first direction and in the seconddirection.

Thereby, by setting the fundamental transverse mode distribution so asto have a large coupling with the gain region in only one direction, itbecomes possible to cause laser oscillation such that the laser beam hasa polarization direction in a specific direction.

Thereby, the higher-order transverse mode has very small electric fieldamplitude in the laser oscillation region and is affected heavily by theantiguiding structure. By providing such a guiding structure, there isachieved only small optical coupling with the gain region for suchhigher-transverse mode, and laser oscillating in higher-transverse modeis effectively suppressed.

Further, it should be noted that the dimensions of various parts notedbefore can be changed according to the needs. Further, the effectiverefractive index in the low refractive index regions 1 and 2 can beadjusted by adjusting the thickness and depth of the Al oxidationregion.

By optimizing the sizes of various parts and the difference of effectiverefractive index such that the coupling between the fundamentaltransverse mode and the gain region becomes large in a specificdirection corresponding to the polarization direction and such that thecoupling is decreased in other directions, the effect of the device ofthe present example can be increased further.

It should be noted that the oxygen ion implantation into the lowrefractive index regions 1 and 2 can be conducted separately in twodifferent steps, by using different acceleration voltages and differention currents.

Thus, with the surface-emission laser diode of the present embodiment,it becomes possible to set the polarization direction arbitrarily bycontrolling the resist pattern at the time of the photolithographicprocess.

Thus, with the surface-emission laser diode of the present embodiment, ahigh output power was obtained in a single fundamental transverse modelaser oscillation, with polarization direction in the first direction.

EXAMPLE 8

FIGS. 13A and 13B are diagrams showing the surface-emission laser diodeof Example 8, wherein it should be noted that FIG. 13B shows a plan viewof the laser diode of FIG. 13A. The surface-emission laser diode ofFIGS. 13A and 13B is a laser diode having an active layer of GaInAs/GaAsmultiple quantum well structure and operating in the 0.98 μm wavelengthband.

Hereinafter, the device structure will be explained together with thefabrication process thereof. The laser diode of FIGS. 13A and 13B aregrown with similar methods and similar means to the case of Example 4.

More specifically, the device of FIGS. 13A and 13B is constructed on ann-GaAs substrate 401 carrying thereon a n-GaAs buffer layer 402, whereinthe n-GaAs buffer layer 402 carries thereon a lower semiconductordistributed Bragg reflector 403 including therein 36 repetitions of ann-Al_(0.9)Ga_(0.1)As/GaAs pair. Further, an undoped GaAs cavity spacerlayer 404 is formed on the lower semiconductor distributed Braggreflector 403, and an active layer 405 of GaInAs/GaAs multiple quantumwell structure is formed on the GaAs cavity spacer layer 404.

On the active layer 405, there is provided another cavity spacer layer406 of undoped GaAs, and an upper distributed Bragg reflector 407including therein 20 repetitions of a p-Al_(0.9)Ga_(0.1)As/GaAs pair isformed further on the cavity spacer layer 406.

Further, there is provided a contact layer (not shown) in correspondenceto the GaAs layer at the uppermost part of the upper distributed Braggreflector 407 such that there is provided a high concentration doping ofp-type dopant (carbon) at the surface part of such a GaAs layer.

After the crystal growth process, there are formed a square resistpattern having a resist opening in correspondence to the low refractiveindex region 1 of FIG. 13B and a resist pattern having a resist openingin correspondence to the low refractive index region 2 of FIG. 13B byusing a known photolithographic process. Further, oxygen ionimplantation is conducted through the surface of theP-Al_(0.9)Ga_(0.1)As/GaAs upper semiconductor distributed Braggreflector while using the resist pattern as a mask.

Next, a thermal annealing process is conducted and there is achieved aselective oxidation process in correspondence to where the ionimplantation of oxygen has been made. Thereby, the low refractive indexregion 1 serving for the antiguiding structure is formed at the centralpart of the device and the low refractive index region 2 (claddingregion) is formed at the peripheral part of the device for confining thetransverse mode.

Here, it should be noted that the opening d1 of the resist patterncorresponding to the low refractive index region 1 is set to have a sizeof 5 μm. Further, the low refractive index region 2 (cladding region) isformed in pair in only one direction (first direction) so as to sandwichthe laser cavity region laterally. Thereby, the region laterallysandwiched by the low refractive index cladding regions has a width d2set to have a size of 30 μm.

FIG. 13A also shows the effective refractive index profile in thedirection parallel to the substrate surface.

Referring to FIG. 13A, it can be seen that the effective refractiveindex is reduced relatively in correspondence to the part where the lowrefractive index region is provided. Further, Further, it can be seenthat there is formed a high refractive index region at both sides of thelow refractive index core at the central part of the device. Further, itcan be seen that there is formed a cladding region of low refractiveindex region at further outer side of the high refractive index region.

FIG. 13B shows the effective refractive index in the second direction.In this direction, there is provided no cladding region, and only thecore part at the central part of the device forms the low refractiveindex region.

After this, there is formed a square resist pattern in alignment withthe oxygen ion implantation part, and ion implantation of hydrogen isconducted into the device structure while using the resist pattern as amask, such that the hydrogen distribution peak is located at a leveldeeper than the oxygen distribution peak in the oxygen distributionpart. With this, the resistance of the layered structure is increased incorrespondence to the hydrogen ion injection part. In the presentexample, the resist pattern is formed to have the edge size d3 of 8 μm.

It should be noted that the foregoing high resistance region can beformed also by ion implantation of oxygen.

Next, a p-side electrode 311 is formed on the device surface and ann-side electrode 312 is formed on the rear surface of the substrate 301after polishing of the rear surface. Further, by conducting an annealingprocess, the p-side electrode 311 and the n-side electrode 312 form anohmic contact. With this, the surface-emission laser diode of FIGS. 13Aand 13B is obtained.

With the surface-emission laser diode of the present example, the lowrefractive index regions 2 (cladding regions) is provided in pair so asto oppose with each other across the laser oscillating region in onlyone direction.

As explained before, the loss by mode leakage is decreased in the firstdirection in which the low refractive index cladding region is provided.

On the other hand, in the second direction in which no such lowrefractive index cladding layer is provided, there occurs a loss as aresult of mode leakage.

Further, in the second direction, the mode distribution is spread in thedirection perpendicular to the mode cavity direction, and the couplingwith the gain region is decreased as compared with the first direction.With regard to the higher-order transverse mode, the coupling with thegain region is reduced significantly in both of the first and seconddirections as a result of existence of the antiguiding structure.

Thus, with the laser diode of the present example, it is possible tocause laser oscillation selectively in the first direction in which thefundamental transverse mode has a large electric field amplitude and alarge coupling is secured with regard to the gain region. Further, withthe surface-emission laser diode of the present embodiment, thedirection of polarization can be controlled arbitrarily by controllingthe resist pattern at the time of the photolithographic process.

According to the device of the present example, it becomes possible toobtain a single fundamental mode laser oscillation such that the laserbeam has a polarization direction in one direction.

EXAMPLE 9

FIGS. 14A and 14B are diagrams showing the surface-emission laser diodeof Example 9, wherein it should be noted that FIG. 14B shows a plan viewof the laser diode of FIG. 14A. The surface-emission laser diode ofFIGS. 14A and 14B is a laser diode having an active layer of GaInAs/GaAsmultiple quantum well structure and operating in the 0.98 μm wavelengthband.

Hereinafter, the device structure will be explained together with thefabrication process thereof. The laser diode of FIGS. 14A and 14B aregrown with similar methods and similar means to the case of Example 1.

More specifically, the device of FIGS. 14A and 14B is constructed on ann-GaAs substrate 401 carrying thereon a n-GaAs buffer layer 402, whereinthe n-GaAs buffer layer 402 carries thereon a lower semiconductordistributed Bragg reflector 403 including therein 36 repetitions of ann-Al_(0.9)Ga_(0.1)As/GaAs pair. Further, an undoped GaAs cavity spacerlayer 404 is formed on the lower semiconductor distributed Braggreflector 403, and an active layer 405 of GaInAs/GaAs multiple quantumwell structure is formed on the GaAs cavity spacer layer 404.

On the active layer 405, there is provided another cavity spacer layer406 of undoped GaAs, and an upper distributed Bragg reflector 407including therein 20 repetitions of a p-Al_(0.9)Ga_(0.1)As/GaAs pair isformed further on the cavity spacer layer 406.

Further, there is provided a contact layer (not shown) in correspondenceto the GaAs layer at the uppermost part of the upper distributed Braggreflector 407 such that there is provided a high concentration doping ofp-type dopant (carbon) at the surface part of such a GaAs layer.

Within the P-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As upper semiconductordistributed Bragg reflector 407, there is provided a selectivelyoxidizing layer 408 of p-AlAs for current confinement, wherein thep-AlAs selectively oxidizing layer 408 is formed with an oxidized region(shown in black) as a result of selective oxidation conducted in thehigh temperature waver vapor ambient starting from the mesa sidewallsurface formed by etching.

After the crystal growth process, there is formed a square resistpattern having a resist opening having a size of 30 μm in thesurface-emission laser diode of FIGS. 14A and 14B, wherein there isformed a square mesa in the layered structure by conducting an etchingstarting from the top surface of thep-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As upper semiconductordistributed Bragg reflector 407 down to a mid position of then-Al_(0.9)Ga_(0.1)As/Al_(0.15)Ga_(0.85)As lower semiconductordistributed Bragg reflector 403 by using a known dry etching process.

Next, there is formed a rectangular resist opening pattern in alignmentwith the central part of the mesa structure in correspondence to the lowrefractive index region 1 of FIG. 14B, and ion implantation of oxygenmolecules is conducted into the central part of the mesa structure.Here, the resist pattern has an edge length d6 (longer edge length) of20 μm in the first direction and an edge length d5 (shorter edge length)of 15 μm in the second direction.

After removing the resist pattern, the p-AlAs selective oxidizing layer414 in the layered structure is subjected to the selective oxidationprocess conducted in a high temperature water vapor ambient such thatthe selective oxidation process proceeds in the direction parallel tothe substrate starting from the mesa sidewall surface toward the centralpart of the mesa structure, and as a result, there is formed aselectively oxidized current confinement structure.

At the same time with the thermal annealing process associated with theselective oxidation process conducted in the high temperature watervapor ambient, there also proceeds oxidation of the Al_(0.9)Ga_(0.1)Asmixed crystal in the region 413 where the oxygen ions have beenintroduced, and as a result, there is formed a selective oxidationregion of relatively low refractive index in the central part of themesa structure as a result of the oxygen ion implantation process. Here,it should be noted that the current confinement region (non-selectivelyoxidized region) may have the edge length d3 of 10 μm.

Next, an SiO₂ layer 409 and an insulation resin layer 410 are formedconsecutively on the entire wafer surface, and a p-side electrode 411 isformed on the insulation resin layer 410 in contact with the top surfaceof the mesa structure. Further, the rear surface of the n-GaAs substrate401 is polished and an n-side electrode 412 is provided thereon.Further, ohmic contact is achieved for each of the electrodes 411 and412 by conducting a thermal annealing process.

With the anisotropic shape of the low refractive index region 1 in thesurface-emission laser diode of the present embodiment, it becomespossible to change the oscillation mode distribution in the directionalong the longer edge and in the direction along the shorter edge.

Thus, by setting the fundamental transverse mode distribution to achievea large coupling with the gain region in a specific direction, itbecomes possible to cause laser oscillation of the fundamentaltransverse mode such that the laser beam has a polarization direction insuch a specific direction.

Here, it should be noted that the dimensions of various parts notedabove can be changed variously. Further, the effective refractive indeedin the low refractive index core can be adjusted in terms of thethickness and depth of the Al oxide region. By adjusting the size of themesa and the low refractive index core, and by adjusting the effectiverefractive index appropriately such that there occurs a large couplingbetween the fundamental transverse mode and the gain region in thedirection of the desired polarization direction and such that couplingbetween the fundamental transverse mode and the gain region is small inother directions, the effect of the present invention can be enhancedfurther.

With regard to the higher-order transverse mode, the electric fieldamplitude is small in the laser oscillation region, and thus, thehigher-order transverse mode is strongly affected by the antiguidingstructure. Thus, coupling with the gain region is inherently weak insuch a higher-order transverse mode, and laser oscillation inhigher-order transverse mode can be suppressed effectively by providingthe antiguiding structure.

Further, with the surface-emission laser diode of the presentembodiment, the polarization direction can be set in an arbitrarydirection by controlling the resist pattern at the time of thephotolithographic process.

Further, while the present example has been explained for the case inwhich the low refractive index region 1 of the surface-emission layerdiode has a rectangular shape, it is also possible to use an ellipticalshape for the low refractive index region 1. In any of these cases,similar effect can be achieved. Further, it is possible to user thestructure of the hydrogen ion implantation type laser diode explainedwith reference to Examples 6-8 in place of the structure of theselectively oxidized laser diode.

With the surface-emission laser diode of the present embodiment, asingle fundamental transverse mode laser oscillation has been achievedin the first polarization direction up to a high output state.

While explanation has been made for various examples above with regardto the case of using an MOCVD process for the crystal growth process, itis also possible with the present invention to use a molecular beamcrystal growth (MBE) process or other crystal growth processes. Further,it is possible to use a semi-insulating substrate or a p-type substratein place of the n-type substrate. Further, the laser oscillationwavelength is not limited to the foregoing 0.85 μm band or 1.3 μm band,and the laser diode can oscillate with the wavelength of 0.65 μm band,0.98 μm band, 1.5 μm band, or the like.

According to the oscillation wavelength, it is possible to use othermaterials for the laser diode. For example, it is possible to use anAlGaInP mixed crystal for the 0.65 μm band. For the 0.98 μm band, it ispossible to use an InGaAs mixed crystal. Further, a GaInNAs(Sb) mixedcrystal can be used for the 1.5 μm band. Thereby, materials transparentin such a wavelength band are used for the distributed Bragg reflectorwith such a combination so as to maximize the refractive indexdifference therebetween.

Further, the device structure may be different from the structureexplained with reference to the foregoing examples. Further, the devicesexplained with the foregoing examples may be tuned to oscillate withother oscillation wavelengths. By choosing the materials andconstruction of the distributed Bragg reflector appropriately accordingto the desired oscillation wavelength, any of the devices explainedabove can be tuned to the desired oscillation wavelength.

In order to reduce the device resistance further, it is effective toprovide a heterospike buffering layer between the Al(Ga)As/GaAsheterointerface with a composition intermediate therebetween. Further,such a heterospike buffering layer may be provided to the interface ofthe selective oxidizing layer.

With regard to the heterospike buffering layer, it is possible to use asingle layer having a composition intermediate of the two layersconstituting the heterointerface or combination of plural layers ofdifferent compositions. Further, it is also possible to change thecomposition continuously.

EXAMPLE 10

FIG. 15 shows a surface-emission laser array according to Example 10 ofthe present invention. More specifically, FIG. 15 shows the constructionof a monolithic laser array-in which the surface-emission laser diodesof the present invention are arranged two-dimensionally in the 4×4formation in a top plan view.

In FIG. 15, there is provided wirings on the upper electrodesindividually so as to drive the laser diodes independently. Thesurface-emission laser diode array of FIG. 15 is formed by the processand method similar to the one used in any of the examples explainedbefore.

Because the higher-order transverse mode laser oscillation iseffectively suppressed in each of the devices constituting thesurface-emission laser array of FIG. 15 by the antiguiding structure oflow refractive index layer formed in the cavity structure by oxygen ionimplantation and subsequent thermal annealing process as shown inExamples 1 through 4, the surface-emission laser array of FIG. 15 canprovide high output power while maintaining the single fundamentaltransverse mode laser oscillation.

Particularly, with the surface-emission laser array in which theantiguiding structure is provided inside the n-type distributed Braggreflector, the device resistance is decreased and a very high outputoperation is possible. Thereby, it becomes possible to obtain asurface-emission laser array operating in the single fundamentaltransverse mode with high output power and with reduced heat generation.

Thus, a surface-emission laser array operating in the single fundamentaltransverse mode with high output power has been obtained.

EXAMPLE 11

FIG. 16 is a diagram showing an example of a surface-emission lasermodule, while FIG. 17 is a diagram showing an example of a paralleloptical interconnection system (Example 11) that connects differentdevices.

The laser array module of FIG. 16 is constructed by mounting aone-dimensional monolithic surface-emission laser array of the presentinvention, a microlens array and a fiber array on a silicon substrate.

Here, the surface-emission laser array is provided in the directiontoward the fibers and are coupled with the silica single mode fibersmounted on V-shaped grooves formed on the silicon substrate via themicrolens array. The surface-emission laser array has an oscillationwavelength of 1.3 μm band and achieves high speed transmission over thesilica single mode fibers.

With the interconnection system of FIG. 17, a device 1 and a device 2are connected by an optical fiber array. Thereby, the device 1 at thetransmission side includes a one-dimensional laser array module thatuses the surface-emission laser diode or the surface-emission laserarray of the present invention and a driving circuit thereof. Further,the device 2 at the reception side includes a photodiode array moduleand a signal detection circuit.

With the optical interconnection system of Example 11, a stablefundamental transverse mode laser oscillation is obtained with regard tothe environmental temperature change or change of driving condition as aresult of use of the surface-emission laser array of the presentinvention. Because there occurs little change of coupling efficiencywith the optical fibers, it becomes possible with the present inventionto construct a highly reliable interconnection system.

While Example 11 has been explained with regard to the parallel opticalinterconnection system, it is also possible to construct a serialtransmission system by using a single device. Further, it is alsopossible to apply the system of Example 11 to inter board connection,inter chip connection and intra chip connection.

EXAMPLE 12

FIG. 18 shows an optical communicating system according to Example 12for the case the optical communication system is used as an optical LAN.

With the optical LAN system of FIG. 18, the surface-emission laser diodeof surface-emission laser array of the present invention is used for theoptical source of optical transmission between a server and a coreswitch, between a core switch and individual switches, and betweenindividual switches and individual terminals. Thereby, connectionbetween the devices is achieved by using a silica single mode fiber or amultimode fiber.

By using the surface-emission laser diode or surface-emission laserarray of the present invention for the optical source, in which thesurface-emission laser diode or surface-emission laser array of thepresent invention is used for the optical source, stable fundamentaltransverse mode laser oscillation is achieved in spite of environmentaltemperature change or change of drive condition, and it became possibleto construct a highly reliable system because of little change ofcoupling efficiency with optical fibers.

EXAMPLE 13

FIG. 19 is a diagram showing an elector-photographic system according toExample 13 of the present invention.

The electrophotographic system of FIG. 19 comprises a photosensitivedrum, scanning and converging optical system (optical scanning system),a writing optical source and a synchronization control part, wherein thesurface-emission laser diode or the surface-emission laser array of thepresent invention is used for the wiring optical source.

The electrophotographic system of FIG. 19 is controlled by asynchronization control circuit, and the optical beam from the opticalsource is focused upon the photosensitive drum by the scanning andconverging optical system that includes a polygonal mirror. Thereby,there is formed an electrostatic latent image on the photosensitivedrum.

Conventionally, it has been difficult with the surface-emission laserdiode to operate with high output power because of extensive heatgeneration, while in the case of the surface-emission laser diode of thepresent invention, operation with higher output power becomes possibleas compared with the conventional device. Thus, the surface-emissionlaser diode of the present invention is suitable for the writing opticalsource of the electrophotographic system.

Further, because the laser oscillation is in the single fundamentaltransverse mode, a single-peak far field image is obtained, and focusingof the optical beam is achieved easily. With this, a high definitionimage is obtained with the present invention.

Further, with the red surface-emission laser diode that uses the AlGaInPmaterial for the active layer, laser oscillation at about 650 nm ispossible, while this oscillation wavelength is shorter than the case ofusing an AlGaAs material. Thereby, the tolerance of design in theoptical system is increased. Thus, such a red surface-emission laserdiode is suitable for the writing optical source of high definitionelectrophotographic systems.

It should be noted that such a surface-emission laser diode can beconstructed by using the material of AlGaInP system for the active layerand by using the material of AlGaAs or AlGaInP system for thedistributed Bragg reflector.

Further, it is possible to achieve crystal growth in lattice matchingwith the GaAs substrate with the use of such a material, and thus, it ispossible to use such an AlAs material for the selective oxidation layer.

On the other hand, with the use of AlGaInP material, there arises aproblem that the laser diode becomes extremely susceptible totemperature change, and associated with this, problems such assaturation of output power or failure of laser oscillation are causedwith temperature increase associated with device heat generation.

With the surface-emission laser diode fabricated with the presentinvention, the higher-order transverse mode distribution is shiftertoward the mesa sidewall surface as a result of use of the antiguidingstructure, while such a shift decreases the degree of coupling of thehigher-order transverse mode distribution with the gain region. Thereby,laser oscillation with higher-order transverse mode is effectivelysuppressed, while this enables use of larger diameter for the currentconfinement region.

Thus, with the present invention, it becomes possible to decrease thediameter of the current confinement region, which has been a problemwith a red surface-emission laser diode, and it becomes possible torealize a device of low resistance.

Thus, with the present invention, it becomes possible to realize a redsurface-emission laser diode of reduced heat generation and capable ofoscillating in the single fundamental transverse mode with higher outputpower as compared with the conventional device, while such a laser diodeis quite suitable for the writing optical source of electrophotographicsystem.

Further, as a result of use of the surface-emission laser array ofExample 10, it becomes possible to increase the writing speed ascompared with the conventional device. Thereby, it becomes possible withthe present invention to obtain a high speed and high definitionelectrophotographic system.

EXAMPLE 14

FIG. 20 is a diagram showing an optical disk system according to Example14 of the present invention.

The optical disk system of FIG. 20 is formed of an optical disk, anoptical system 1, an optical system 2, a beam splitter, an opticaldetector, a laser optical source, and a synchronization part(synchronization control circuit), wherein the surface-emission laserdiode or surface-emission laser array of the present invention is usedfor the laser optical source.

Here, it should be noted that the optical system 1, optical system 2,beam splitter, optical detector and the laser optical source constitutean optical head, wherein the optical head is driven by an actuator andachieves access to an arbitrary track on the disk. Here, the opticalsystem 1 is constructed by a diffraction grating and a beam expansionlens, while the optical system 2 is constructed of a ¼ wavelength plateand a beam converging lens.

With the optical disk system of FIG. 20, the laser beam from the lasersource is focused upon the disk surface by the optical source 1 and theoptical source 2 under control of the synchronization circuit, and thedisk surface is irradiated with the laser beam.

On the disk surface, there is formed a track by information pitsarranged in the form of a regular array, and the laser beam reflectedfrom such a disk surface is directed to the optical detector in thereading operation by the beam splitter after passing through the opticalsystem 2 again. In the optical detector, the information signal and alsothe tracking signal formed by the information pits are detected, andservo control of the optical head is achieved based on the detectedsignals with regard to the distance between the disk and the head andwith regard to the head and the track.

With the surface-emission laser diode of the present invention, a largeroutput power is possible as compared with the conventional device whilemaintaining the single fundamental transverse mode operation, andbecause of this, a single peak beam spot is obtained stably. Thereby,the optical system necessary for beam shaping is simplified, and thecost of the optical disk system is reduced.

Because it becomes possible to obtain a single peak beam spot withreliability, the optical disk system constructed by using thesurface-emission laser diode of the present invention provides excellentreliability. Further, it becomes possible to achieve high speed readingas a result of use of the surface-emission laser array of Example 10.

Thus, with the present invention, it becomes possible to obtain a highspeed and highly reliable optical disk system.

Hereinafter, further best modes for implementing the present inventionwill be explained.

In the present invention, anisotropy is provided for confinement oftransverse mode in a surface-emission laser diode having an antiguidingstructure for controlling the polarization direction thereof.

(Nineteenth Mode of Invention)

According to the nineteenth mode of the present invention, there isprovided a surface-emission laser diode constructed on a substratehaving a substrate surface, comprising:

-   -   an active layer parallel to said substrate surface;    -   a pair of cavity spacer layers provided so as to sandwich said        active layer;    -   a pair of distributed Bragg reflectors provided across said        active layer and said cavity spacer layers so as to sandwich        said active layer and said cavity spacer layers therebetween;    -   a high resistance region defining a current injection region for        injecting a current into said active layer,    -   laser oscillation being caused in a laser cavity region between        said pair of distributed Bragg reflectors acting as cavity        mirrors in a direction perpendicular to said substrate surface        in correspondence to said current injection region,    -   a refractive index structure provided in a plane parallel to        said substrate surface, said refractive index structure        comprising: a low refractive index core including said laser        cavity region at a central part thereof; and a periodic        structure provided around said low refractive index core, said        periodic structure comprising a low refractive region        surrounding said low refractive index core and a high refractive        index region surrounding said low refractive index region, said        low refractive index region and said high refractive index        region being repeated alternately in said plane parallel to said        substrate,    -   wherein any of a width of said low refractive index core or a        shape of said periodic structure is changed between a specific        direction parallel to said substrate surface and other        directions parallel to said substrate surface different from        said specific direction.

Thus, with the present invention, any of the width of the low refractiveindex core or the shape of the periodic structure is changed in aspecific direction and in other directions within the plane parallel tothe substrate surface. Thereby, there is caused anisotropy in the widthof the low refractive index core or the shape of the periodic structurebetween the foregoing specific direction and directions other than thespecific direction.

Here, the low refractive index core may be the laser oscillation regioncorresponding to the current injection region.

Further, the width of the low refractive index core or the shape of theperiodic structure may change in various, mutually different directionsother than the foregoing specific direction.

In the description hereinafter, it should be noted that therepresentation “periodic structure of refractive index provided in thedirection parallel to the substrate surface” or “periodic structure ofrefractive index” means the periodic structure in which the region oflow effective refractive index and the region of high refractive indexare repeated in the direction perpendicular to the laser cavitydirection.

According to the nineteenth mode of the present invention, in whichthere is provided a surface-emission laser diode constructed on asubstrate having a substrate surface, comprising: an active layerparallel to said substrate surface; a pair of cavity spacer layersprovided so as to sandwich said active layer; a pair of distributedBragg reflectors provided across said active layer and said cavityspacer layers so as to sandwich said active layer and said cavity spacerlayers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein any of a width of said low refractive index core or ashape of said periodic structure is changed between a specific directionparallel to said substrate surface and other directions parallel to saidsubstrate surface different from said specific direction, there iscaused anisotropy in the width of the low refractive index core or inthe shape of the periodic structure between a specific direction anddirections different from the foregoing specific direction, while thisinduces anisotropy in the strength of transverse mode confinement causedby the periodic structure of the refractive index provided within theplane parallel to the substrate. Thereby, it becomes possible to causeselective oscillation of the fundamental transverse mode, such that thefundamental transverse mode has a polarization plane in specificdirection.

Non-Patent Reference 6 noted before describes about confinement of thetransverse mode by antiguiding structure (ARROW structure) and aperiodic structure of the refractive index provided perpendicularly tothe laser cavity direction.

Here, the antiguiding structure means a structure forming a waveguidestructure of transverse mode provided in the region where there occurslaser resonance in the plane parallel to the substrate surface, suchthat there occurs a relative decrease of effective refractive index ascompared with the surrounding region (high refractive index cladding).Further, Non-Patent Reference 6 discloses an example of the periodicstructure provided in the high refractive index cladding layer such thatthe region of low effective refractive index and the region of higheffective refractive index are repeated within the plane parallel to thesubstrate surface for reducing the diffraction loss of the transversemode.

As described in Non-Patent Reference 6, the wavelength λ1 in thedirection parallel to the substrate surface in the high refractive indexcladding can be approximated, in the antiguiding structure formed by asingle low refractive index core and single high refractive indexcladding, as $\begin{matrix}{\lambda_{1} = \frac{\lambda_{0}}{\left\{ {n_{1}^{2} - n_{0}^{2} + \frac{\left( {m + 1} \right)^{2}\lambda_{0}^{2}}{4d^{2}}} \right\}^{\frac{1}{2}}}} & (1)\end{matrix}$wherein the low refractive index core has the refractive index n₀, thehigh refractive cladding has the refractive index n₁, the optical beamhas the wavelength λ₀ in vacuum, and the low refractive index core hasthe width d. Here, it should be noted that the wavelength in thedirection parallel to the substrate surface represents the wavelengthcorresponding to the propagation coefficient (wavenumber) of the opticalwave in the direction parallel to the substrate surface and means thewavelength of the optical wave propagating in an arbitrary direction asit is projected on the substrate surface.

Further, m in Equation (1) represents the order of the transverse mode.

Thus, the wavelength λ₁ in the direction parallel to the substratesurface is determined by the refractive index n₀ of the low refractiveindex core, the refractive index n₁ of the high refractive indexcladding and the width d of the low refractive index core.

In the case the refractive indices n₀ and n₁ of the low refractive indexcore and the high refractive index cladding are constant, the wavelengthλ₁ depends on the width d of the low refractive index core as can beseen in Equation (1).

Thus, in the case the high refractive index cladding has a uniformrefractive index, there occurs a change in the wavelength λ₂ in thedirection parallel to the substrate surface according to the width d ofthe low refractive index core. As described in Non-Patent Reference 2,the wavelength λ₁ for the zero-th order, first order and second ordertakes the values of 1.70 μm, 1.66 μm and 1.62 μm in the case of n₀=3.3,n₁=3.35, λ₀=0.98 μm and d=8 μm.

Particularly, by choosing the width of the regions constituting theperiodic structure provided in the direction parallel to the substratesurface to be equal to an odd number multiple of ¼ of the lateralwavelength (wavelength in the direction parallel to the substratesurface) of the transverse mode in respective regions, it becomespossible to provide a cavity structure similarly to the cavity structureprovided in the laser cavity direction. In this case, in which thecavity part thus formed has a relatively low refractive index, there isformed a half wavelength resonator in which the electric field strengthof the standing wave becomes zero at the interface between the cavitypart and the periodic structure of the refractive index.

With such a construction, the fundamental transverse mode is confined inthe low refractive index region by the periodic structure of therefractive index, and mode leakage in the direction parallel to thesubstrate is suppressed. Thereby, mode loss can be reduced.

Here, it should be noted that, by choosing the number of repetitions inthe periodic structure of the refractive index to be small, the width ofthe reflection band (stop band) is decreased. Thereby, in view of thefact that fact that the higher order transverse modes have a relativelylarge wavelength separation of about 40 nm with respect to thefundamental transverse mode, the resonance wavelengths for thehigher-order transverse modes are made outside of the stop band withcertainty, and it becomes possible to confine selectively thefundamental transverse mode in the direction parallel to the substratesurface.

It should be noted that such a region of different refractive index inthe direction perpendicular to the laser cavity direction (directionparallel to the substrate) can be formed by changing the resonantwavelength in the vertical direction to the laser cavity directionwithin the plane perpendicular to the laser cavity direction.

More specifically, it is known that there exists an equivalentrelationship $\begin{matrix}{\frac{\Delta\quad n}{n} = \frac{\Delta\lambda}{\lambda}} & (2)\end{matrix}$between the effective refractive index change Δn and the resonantwavelength change Δλ in the direction parallel to the substrate surface,and thus, it becomes possible to increase the effective refractive indexrelatively by increasing the resonant wavelength relatively to otherregions and it becomes possible to decrease the effective refractiveindex by decreasing the resonant wavelength with respect to otherregions.

In order to cause a large change in the resonant wavelength, it iseffective to change the thickness of the layer in the vicinity of thelaser cavity region such as the cavity spacer layer. In the device ofNon-Patent Reference 2, there is formed a region of relatively loweffective refractive index with regard to the surrounding region bydecreasing the thickness of a specific semiconductor layer in theforegoing specific region relatively to the surrounding region byetching.

Thus, with the foregoing conventional art, it becomes possible todecrease the mode leakage in the direction parallel to the substratesurface by providing a periodic region of refractive index formed of aregion of low effective refractive index and a region of high effectiverefractive index around the low refractive index core. Further, itbecomes possible to confine the fundamental transverse mode selectively,and it becomes possible to obtain a high output power operation in thesingle fundamental transverse mode.

Next, the construction and operation of the surface-emission laser diodeaccording to the nineteenth mode of the present invention will bedescribed.

In the surface-emission laser diode of the present invention, theconventional surface emission laser diode having an antiguidingstructure and provided with a low refractive index core and a periodicstructure of refractive index surrounding the low refractive index inthe direction parallel to the substrate surface is improved by providinganisotropy in the width of the low refractive index core or in the shapeof the periodic structure in the direction parallel to the substratesurface. Thereby, the degree of transverse mode confinement is changedin the anisotropic directions, and thus, it becomes possible to controlthe polarization direction of the laser beam in a specific direction,while maintaining the single fundamental transverse mode operation up tohigh output power state.

Thus, the wavelength in the lateral direction in the antiguidingstructure is determined by the difference of refractive index betweenthe low refractive index region and the high refractive index region orthe width of the low refractive index core, while the reflectivity ofthe reflection wavelength band (stop band) formed by the periodicstructure is determined based on the width of the low refractive indexregion and the high refractive index region and also by the repetitionperiod of the periodic structure.

Thus, by providing anisotropy in the width or shape of the periodicstructure in the direction parallel to the substrate surface, it becomespossible to set the degree of mode confinement in the direction parallelto the substrate surface to different values. By setting the degree oftransverse mode confinement to have different values, it becomespossible to change the loss caused by mode leakage in the directionparallel to the substrate. Further, it becomes possible to change thecoupling between the mode distribution and the gain region correspondingto the current injection region.

Thus, by choosing the foregoing periodic structure such that thereoccurs a large mode leakage loss in a particular direction, or bychoosing the periodic structure such that the coupling between the modedistribution and the gain region is decreased in a particular direction,it becomes possible to suppress the laser oscillating in the mode thathas an electric field amplitude (polarization) in such a direction.

Thus, with the nineteenth mode of the present invention, it becomespossible to obtain high output power operation in single fundamentaltransverse mode by using the periodic structure formed of the highrefractive index region and the low refractive index region. Further, itbecomes possible to control the polarization direction in a specificdirection by providing anisotropy in the width of the low refractiveindex core or in the construction of the periodic structure in thedirection parallel to the substrate surface.

In the surface-emission laser diode of the present mode, it ispreferable to use a high resistance region formed by hydrogen ionimplantation for the current confinement structure. With the use of thehigh resistance region formed by hydrogen ion implantation, there occurslittle refractive index change and designing of the periodic structureof refractive index can be made easily.

Further, with the use of the high resistance region formed by hydrogenion implantation for the current confinement structure, there is no needof selective oxidation processing, and thus, there is no need of forminga mesa structure with the laser diode of such a type. Because of theabsence of the mesa structure, the laser diode shows excellentperformance of heat radiation in the lateral direction.

Further, because the surface-emission laser diode of the present modeachieves the current confinement and transverse mode control bydifferent structures, there is no need of narrowing the currentconfinement diameter for the single fundamental transverse mode controlcontrary to the case of the conventional selective oxidation typesurface-emission laser diode, and a device of low resistance can beobtained easily.

Further, because the device does not include a selectively oxidizedcurrent confinement layer in the layered structure constituting thedevice, there occurs no problem of parasitic capacitance.

Further, as compared with the conventional surface-emission laser diodeof the hydrogen ion implantation type, the laser diode of the presentmode is improved with regard to the problem of unstable transverse mode.Further, because of the low device resistance and small parasiticcapacitance, the surface emission laser diode of the present mode showsexcellent electric characteristics, particularly the frequencycharacteristics. Further, because of the small heat generation and highefficiency of heat radiation, large differential gain and high outputpower are achieved with the surface emission laser diode of the presentmode. Thereby, it becomes possible to obtain a high relaxationoscillation frequency.

Thus, with the present mode of the invention, it becomes possible toobtain a surface-emission laser diode capable of high speed modulation.

(Twentieth Mode of Invention)

According to the twentieth mode of the present invention, there isprovided a surface-emission laser diode according to the nineteenthaspect of the present invention, wherein said low refractive index corehas a width different between two directions parallel to said substratesurface and crossing perpendicularly with each other.

With the twentieth aspect of the present invention, in which there isprovided a surface-emission laser diode according to the nineteenthaspect of the present invention (antiguiding surface-emission laserdiode using a low refractive index core for the laser cavity region),wherein said low refractive index core has a width different between twodirections parallel to said substrate surface and crossingperpendicularly with each other, there is induced anisotropy in thedegree of confinement of the transverse mode caused by the periodicstructure of refractive index provided within the plane parallel to thesubstrate, it becomes possible to obtain an operation, in which thefundamental transverse mode having a polarization in a specificdirection is selectively oscillated.

Hereinafter, more detailed description will be made.

As noted before, the wavelength λ₁ in the high refractive index claddinglayer in the direction parallel to the substrate surface is determinedby the width d of the low refractive index core according to Equation(1) in the case the refractive indices n₀ and n₁ of the low refractiveindex core and the high refractive index core are constant.

Thus, in the case the width of the low refractive index core is changedin two, mutually perpendicular directions as in the twentieth mode ofthe present invention, the wavelength in the direction parallel to thesubstrate surface takes different values in the respective directions.

Thus, in the case there is provided the same periodic structure withinthe plane parallel to the substrate surface in the two, mutuallyperpendicular directions as the periodic structure of refractive index,there appears a difference in the degree of transverse mode confinementbetween the two directions perpendicular with each other because of thefact that the wavelength parallel to the substrate surface is differentbetween these two directions.

Thus, when the width and period of the periodic structure is chosen suchthat the confinement of the fundamental transverse mode by the periodicstructure of the refractive index is made most efficiently in one of thetwo, mutually perpendicular directions (referred to hereinafter as“first direction”), then there inevitably arises the situation that thedegree of confinement of the fundamental transverse mode is weakened inthe other direction (referred to hereinafter as “second direction”),because of the difference of wavelength of the fundamental transversemode in the direction parallel to the substrate surface in the seconddirection as compared with the wavelength in the first direction, andthere is caused a large mode leakage in the direction parallel to thesubstrate surface.

Thus, there appears a large loss in the second direction parallel to thesubstrate surface, and it becomes possible to cause laser oscillationselectively such that the laser beam has a polarization in the firstdirection.

Thus, in the case there is provided a periodic structure of differentrefractive indices such that a maximum efficiency of confinement isachieved for the fundamental transverse mode having the wavelengthparallel to the substrate surface and directed in a specific direction(first direction), it becomes possible to cause selective oscillation ofthe fundamental transverse mode having the polarization in the firstdirection by changing the width of the low refractive index core in thedirection (second direction) perpendicular to the foregoing firstdirection.

Further, by confining the fundamental transverse mode selectively, theperiodic structure formed of the high refractive index region and thelow refractive index region maintains the single fundamental transversemode up to high output power state of the laser diode.

With regard to the current confinement structure of the surface-emissionlaser diode of the present mode of the invention, it is preferable touse the high resistance region formed by hydrogen ion implantationprocess similarly to the case of the nineteenth mode of the invention.With this, the operation similar to the one explained with reference tothe nineteenth mode of the invention is attained, and it becomespossible to provide a device of low device resistance, small capacitanceand superior electric characteristics including the frequencycharacteristics.

Because of small heat generation and high efficiency of heat radiation,it becomes possible to obtain a high differential gain and high outputpower. Thereby, it becomes possible to realize a high relaxationoscillation frequency. Thus, a surface-emission laser diode capable ofhigh speed modulation is obtained.

(Twenty First Mode of Invention)

According to the nineteenth aspect of the present invention, there isprovided a surface-emission laser diode according to the nineteenthaspect of the present invention, wherein said periodic structure isdifferent between two directions parallel to said substrate surface andcrossing perpendicularly with each other.

As explained with reference to the nineteenth and twentieth mode of thepresent invention, it is possible with the periodic structure providedaround the low refractive index core to achieve effective confinement ofthe fundamental transverse mode by choosing the width of the lowrefractive index region and the high refractive index regionconstituting the periodic structure to be an odd integer multiple of ¼of the lateral wavelength of the fundamental transverse mode (wavelengthin the direction parallel to the substrate surface) in each of the lowrefractive index region and the high refractive index region. In thissay, the efficiency of confinement of the transverse mode is determinedby the construction of the regions constituting the periodic structuresuch as the width thereof.

Further, as explained before, the wavelength λ₁ in the directionparallel to the substrate surface in the high refractive index claddinglayer is determined by the width d of the low refractive index coreaccording to Equation (1) in the case the refractive index n₀ of the lowrefractive index core and the refractive index n₁ of the high refractiveindex cladding are constant.

Thus, in the case the low refractive index core has an anisotropic shapesuch as a square form, the wavelength λ₁ in the direction parallel tothe substrate surface becomes identical in the two, mutuallyperpendicular directions.

Thus, with the twenty first mode of the present invention, in whichthere is provided anisotropy in the construction of the periodicstructure, it becomes possible to change the degree of confinement ofthe transverse mode according to the directions different in terms ofthe anisotropy.

For example, in the case the width of the high refractive index regionand the width of the low refractive index region are set to an oddinteger multiple of ¼ the wavelength of the fundamental transverse modein the direction parallel to the substrate surface so as to satisfy thephase condition of Bragg reflection such that the confinement of thefundamental transverse mode by the periodic structure of the refractiveindex is achieved in one of the mutually perpendicular two directions(referred to hereinafter as first direction) with highest efficiency inthis first direction, and at the same time the width of the lowrefractive index region and the width of the high refractive indexregion are set to other values in the second direction perpendicular tothe first direction, the Bragg reflection condition is not satisfied inthe second direction. Thereby, the efficiency of transverse mode isweakened in the second direction as compared with the first direction.

Thus, laser oscillation with the mode having the electric fieldamplitude (polarization) in the second direction is suppressed as aresult of the loss caused by mode leakage. Thus, laser oscillationoccurs selectively only in the fundamental transverse mode having thepolarization in the first direction.

Thus, with the twenty first mode of the present invention, it becomespossible to control the polarization direction of the output laser beamin a particular direction by providing anisotropy in the periodicstructure of refractive index provided around the low refractive indexcore. Further, the surface-emission laser diode performs operation up tohigh output power state while maintaining the single fundamentaltransverse mode oscillation.

With regard to the method of providing anisotropy to the periodicstructure, it is also possible to change the refractive index orrepetition pitch of the low refractive index region and the highrefractive index region, in addition to the approach of changing thewidth of the low refractive index region and the width of the highrefractive index region.

With regard to the current confinement structure of the surface-emissionlaser diode of the present mode of the invention, it is preferable touse the high resistance region formed by hydrogen ion implantationprocess similarly to the case of the nineteenth mode of the invention.With this, the operation similar to the one explained with reference tothe nineteenth mode of the invention is attained, and it becomespossible to provide a device of low device resistance, small capacitanceand superior electric characteristics including the frequencycharacteristics.

Because of small heat generation and high efficiency of heat radiation,it becomes possible to obtain a high differential gain and high outputpower. Thereby, it becomes possible to realize a high relaxationoscillation frequency. Thus, a surface-emission laser diode capable ofhigh speed modulation is obtained.

(Twenty Second Mode of Invention)

According to the twenty second mode of the present invention, there isprovided a surface-emission layer diode according to any of thetwentieth or twenty first aspects of the present invention, wherein areflection wavelength band of said periodic structure is set, in one ofsaid two directions crossing perpendicularly with each other, to belonger than a wavelength of a fundamental transverse mode in said samedirection and projected upon said substrate surface.

In order to obtain a high polarization ratio in the surface-emissionlaser diode of the twentieth or twenty first aspect of the presentinvention up to high output power state, it is necessary to suppress thelaser oscillation with the mode having the polarization direction in theforegoing second direction.

For this purpose, it is effective to set the reflection band (stop band)of the periodic structure in the second direction to be relativelylonger than the lateral wavelength (wavelength projected upon thesubstrate surface) of the fundamental transverse mode in the seconddirection.

As can be seen from Equation (1), the lateral wavelength of thehigher-order transverse mode is always shorter than the lateralwavelength of the fundamental transverse mode. Thus, it becomes possibleto realize low reflectivity and large mode leakage to all the modes bysetting the stop band of the periodic structure of refractive index atthe longer wavelength side with respect to the lateral wavelength of thefundamental transverse mode.

With this, laser oscillation with the mode having polarization in thesecond direction is effectively suppressed and high polarization ratiois achieved. The wavelength of the fundamental transverse mode projectedupon the substrate surface can be evaluated from the structure of thesurface-emission laser diode by using Equation (1).

Thus, with the twenty second mode of the invention, it becomes possibleto suppress the laser oscillation in the mode having electric fieldamplitude in the second direction more efficiently, and it becomespossible to obtain a single fundamental transverse mode oscillationhaving a large polarization ratio up to high output power state.

(Twenty Third Mode of Invention)

According to the twenty third mode of the present invention, there isprovided a surface-emission laser diode constructed on a substratehaving a substrate surface, comprising:

-   -   an active layer parallel to said substrate surface;    -   a pair of cavity spacer layers provided so as to sandwich said        active layer;    -   a pair of distributed Bragg reflectors provided across said        active layer and said cavity spacer layers so as to sandwich        said active layer and said cavity spacer layers therebetween;    -   a high resistance region defining a current injection region for        injecting a current into said active layer,    -   laser oscillation being caused in a laser cavity region between        said pair of distributed Bragg reflectors acting as cavity        mirrors in a direction perpendicular to said substrate surface        in correspondence to said current injection region,    -   a refractive index structure provided in a plane parallel to        said substrate surface, said refractive index structure        comprising: a low refractive index core including said laser        cavity region at a central part thereof; and a periodic        structure provided around said low refractive index core, said        periodic structure comprising a low refractive region        surrounding said low refractive index core and a high refractive        index region surrounding said low refractive index region, said        low refractive index region and said high refractive index        region being repeated alternately in said plane parallel to said        substrate,    -   wherein said periodic structure is provided partially in said        plane parallel to said substrate surface in a specific        direction.

Here, the low refractive index core may be the laser oscillation regioncorresponding to the current injection region.

With the twenty third mode of the present invention, in which there isprovided a surface-emission laser diode constructed on a substratehaving a substrate surface, comprising: an active layer parallel to saidsubstrate surface; a pair of cavity spacer layers provided so as tosandwich said active layer; a pair of distributed Bragg reflectorsprovided across said active layer and said cavity spacer layers so as tosandwich said active layer and said cavity spacer layers therebetween; ahigh resistance region defining a current injection region for injectinga current into said active layer, laser oscillation being caused in alaser cavity region between said pair of distributed Bragg reflectorsacting as cavity mirrors in a direction perpendicular to said substratesurface in correspondence to said current injection region, a refractiveindex structure provided in a plane parallel to said substrate surface,said refractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein said periodic structure isprovided partially in said plane parallel to said substrate surface in aspecific direction, it becomes possible to confine the fundamentaltransverse mode selectively and effectively in the direction(hereinafter the direction in which control of polarization direction ismade will be referred to as first direction similarly to nineteenth modeof the invention) in which the periodic structure of the low refractiveindex region and the high refractive index region is provided, and itbecomes possible to obtain the single fundamental transverse mode laseroscillation up to high output power state.

With regard to the second direction (hereinafter, the direction in whichthe laser oscillating is suppressed will be referred to as seconddirection, similarly to the nineteenth mode of invention), in which noperiodic structure is provided, there exists complete antiguidingaction, and in view of the absence of confinement structure of thetransverse mode, there appears a large loss caused by mode leakage inthe direction parallel to the substrate surface. Thereby, laseroscillation is effectively suppressed.

Thus, it becomes possible to obtain a single fundamental transverse modelaser oscillation with polarization in the first direction.

Because the periodic structure of the high refractive index region andthe low refractive index region provided in the first direction confinesonly the fundamental transverse mode selectively, there appears a largeloss with regard to the higher-order transverse mode similarly to thecase of confinement in the second direction.

Thus, it becomes possible to obtain a single fundamental transverse modelaser oscillation up to high output power state with polarizationaligned in the first direction.

With regard to the current confinement structure of the surface-emissionlaser diode of the twenty third mode of the invention, it is preferableto use the high resistance region formed by hydrogen ion implantationprocess similarly to the case of the nineteenth mode through twentysecond of the invention. With this, the operation similar to the oneexplained with reference to the nineteenth mode of the invention isattained, and it becomes possible to provide a device of low deviceresistance, small capacitance and superior electric characteristicsincluding the frequency characteristics.

Because of small heat generation and high efficiency of heat radiation,it becomes possible to obtain a high differential gain and high outputpower. Thereby, it becomes possible to realize a high relaxationoscillation frequency. Thus, a surface-emission laser diode capable ofhigh speed modulation is obtained.

(Twenty Fourth Mode of Invention)

According to the twenty fourth mode of the present invention, there isprovided a surface-emission laser diode according to any of thenineteenth through twenty third modes of the invention, wherein theactive layer is formed of a group III-V compound semiconductor material,the group III element constituting the active layer contains at leastone of Ga and In, and wherein the group V element constituting theactive layer includes one or all of As, N, Sb and P.

Thus, the surface-emission laser diode of the twenty fourth aspect ofthe present invention contains one or all of Ga and In as the group IIIelement constituting the active layer and one or all of As, N, Sb and Pas the group V element.

By using the foregoing material for the active layer, it becomespossible to obtain a surface-emission laser diode having an oscillationwavelength in the range of 1.1 μm to 1.6 μm on a GaAs substrate.

On a GaAs substrate, it is possible to provide a distributed Braggreflector that uses an AlGaAs mixed crystal having excellentcharacteristics, and with this, it becomes possible to obtain a laserdiode of excellent characteristics. Among these materials, GaInNAs inwhich nitrogen is added to GaInAs with an amount of several percent orless is particularly suitable for the active layer of the laser diode inview of it large conduction band discontinuity with regard to thebarrier layer of GaAs, and the like. Because of this, the laser diode ofthe thirteenth aspect of the present invention has a superiortemperature characteristic to the conventional laser diode of the samewavelength band and formed on an InP substrate.

Further, according to the nineteenth through twenty third modes of thepresent invention, the laser diode provides the laser beam withpolarization aligned in a specific direction, and the influence of noisecaused by the polarization-dependent reflectivity of the optical systemis eliminated. Further, the laser diode maintains single transverse modeoscillation up to high output power state, and a large couplingefficiency is realized with regard to an optical fibber, or the like.

Thus, a surface-emission laser diode suitable for optical fibercommunication is obtained.

(Twenty Fifth Mode of Invention)

According to the twenty fifth mode of the present invention, there isprovided a surface-emission laser array formed of a surface-emissionlaser diode according to any of the nineteenth through twenty fourthmodes of the present invention.

Thus, with the surface-emission laser array of the twenty fifth mode ofthe invention, a monolithic laser array is constructed by using thesurface-emission laser diode according to any of the nineteenth throughtwenty fourth modes of the present invention.

According to the twenty fifth mode of the invention, it becomes possibleto form a monolithic array in which the polarization direction isaligned in an arbitrarily set specific direction easily.

Further, according to the fourteenth aspect of the present invention, itbecomes possible to obtain a surface-emission laser array capable ofmaintaining the fundamental transverse mode laser oscillation up to thehigh output state and producing high quality laser beams. Thus, thesurface emission laser array of the present invention is suitable forthe optical source of multiple beam writing system for use in anelectro-photographic system or for the optical source of long-rangeoptical communication system.

Further, with the surface-emission laser array of the present invention,the polarization direction of the individual elements can be set in anarbitrary direction by the resist patterning process at the time of thephotolithographic process, and thus, it becomes possible to obtain asurface-emission laser array in which the polarization direction is setarbitrarily in the individual elements constituting the array.

(Twenty Sixth Mode of Invention)

According to the twenty sixth mode of the present invention, there isprovided an optical interconnection system that uses thesurface-emission laser diode according to any of the nineteenth throughtwenty fourth modes of the present invention or the surface-emissionlaser array according to the twenty fifth mode of the present invention.

Thus, in the optical interconnection system of the twenty sixth mode ofthe invention, the surface-emission laser diode according to any of thenineteenth through twenty fourth modes of the present invention or thesurface-emission laser array according to the twenty fifth mode of thepresent invention is used for the optical source of the opticalinterconnection system.

With the surface-emission laser diode or surface-emission laser array ofthe present invention, the polarization is controlled in a specificdirection, and thus, there occurs no problem of noise caused by theoptical system due to the dependence of the optical system such asreflectivity on the polarization direction.

Further, because the fundamental transverse mode laser oscillation ismaintained up to the high output state, large coupling with opticalfiber is achieved.

Further, because laser oscillation in the higher-order transverse modeis suppressed, there occurs little change of optical coupling with theoptical fiber even in the case the operational state of the laser diodesuch as optical output is changed, and thus, there occurs little changeof optical injection into the optical fiber. Thus, the opticalinterconnection system using the surface-emission laser diode orsurface-emission laser array of the present mode of the invention hashigh reliability.

(Twenty Seventh Mode of Invention)

According to the twenty seventh mode of the present invention, there isprovided an optical communication system that uses the surface-emissionlaser diode according to any of the nineteenth through twenty fourthmodes of the present invention or the surface-emission laser arrayaccording to the twenty fifth mode of the present invention.

Thus, in the optical communication system of the twenty seventh mode ofthe invention, the surface-emission laser diode according to any of thenineteenth through twenty fourth modes of the present invention or thesurface-emission laser array according to the twenty fifth mode of thepresent invention is used for the optical source of the opticalinterconnection system.

With the surface-emission laser diode or surface-emission laser array ofthe present invention, the polarization is controlled in a specificdirection, and thus, there occurs no problem of noise caused by theoptical system due to the dependence of the optical system such asreflectivity on the polarization direction.

Further, because the fundamental transverse mode laser oscillation ismaintained up to the high output state, large coupling with opticalfiber is achieved.

Further, because laser oscillation in the higher-order transverse modeis suppressed, there occurs little change of optical coupling with theoptical fiber even in the case the operational state of the laser diodesuch as optical output is changed, and thus, there occurs little changeof optical injection into the optical fiber. Thus, the opticalcommunication system using the surface-emission laser diode orsurface-emission laser array of the present mode of the invention hashigh reliability.

(Twenty Eighth Mode of Invention)

According to the twenty eighth mode of the present invention, there isprovided an electrophotographic system that uses any of thesurface-emission laser diode according to the nineteenth through twentyfourth modes of the present invention or the surface-emission laserarray according to the twenty fifth mode of the present invention.

Thus, the electrophotographic system of the twenty eighth mode of thepresent invention uses any of the surface-emission laser diode accordingto the nineteenth through twenty fourth modes of the present inventionor the surface-emission laser array according to the twenty fifth modeof the present invention as a writing optical source.

With electrophotographic systems, the degree of tolerance in thedesigning of the optical system is limited and there has been a problemof large influence of the polarization direction on the optical systemsuch as reflectivity. Thus, with the surface-emission laser diode not inwhich control of polarization direction is not made, it has beendifficult to obtain a high definition electrophotographic system.

Contrary to this, the polarization direction is controlled in a specificdirection with the surface-emission laser diode or surface-emissionlaser array of the present invention, and it becomes possible toconstruct a high definition photographic system.

Further, the output power was small with the conventionalsurface-emission laser diode, and it has been difficult to use such asurface-emission laser diode as the writing optical source of theelectrophotographic system.

Contrary to this, laser oscillation of fundamental transverse mode isobtained with high output power with the surface-emission laser diode orsurface-emission laser array of the present invention. Thus, it hasbecome possible to user the surface-emission laser diode orsurface-emission laser array as the writing optical source of anelectrophotographic system with the present invention.

Further, by using the surface-emission laser diode as the writingoptical source of an electrophotographic system, a round or circularoutput laser beam is obtained. This means that shaping of the laser beamis achieved easily.

Further, because of inherently high positional alignment between thelaser diode elements in the array, it becomes possible to focus plurallaser beams easily and with excellent reproducibility by using a singlelens. Thereby, the construction of the optical system is simplified, andit becomes possible to obtain a high-definition system with low cost.

Because the surface-emission laser diode of the present invention has alarge output power, it becomes possible to achieve high speed writingparticularly in the case a laser array is used.

Thus, with the present invention, it becomes possible to provide ahigh-definition electrophotographic system.

(Twenty Ninth Mode of Invention)

According to the twenty ninth mode of the present invention, there isprovided an optical disk system that uses any of the surface-emissionlaser diode according to the nineteenth through twenty fourth modes ofthe present invention or the surface-emission laser array according tothe twenty fifth mode of the present invention.

Thus, the optical disk system of the twenty ninth mode of the presentinvention uses any of the surface-emission laser diode according to thenineteenth through twenty fourth modes of the present invention or thesurface-emission laser array according to the twenty fifth mode of thepresent invention as a read/write optical source.

Among various optical disk systems, a magneto-optical disk requires alaser source having linear polarization for reading signals. Because thesurface-emission laser diode of the present invention provides linearpolarization in which the polarization direction is aligned in aspecific direction, it becomes possible to use a surface-emission laserdiode for the reading optical source of the magneto-optical disk, inwhich the use of surface-emission laser diode has hitherto beendifficult.

Further, because of small output power in the single fundamentaltransverse mode laser oscillation, it has been difficult to use aconventional surface-emission laser diode for the optical source of anoptical disk system.

With the surface-emission laser diode or surface-emission laser array ofthe present invention, stable fundamental transverse mode laseroscillation is obtained up to high output power state. Thus, it becomespossible to use a surface-emission laser diode or surface-emission laserarray as the writing optical source of an optical disk system with thepresent invention, and it becomes possible to construct a reliableoptical disk system. By using a surface-emission laser array, it becomespossible to achieve high-density reading and writing becomes possible,and it becomes possible to construct a high-speed optical disk system.

Thus, with the present invention, it becomes possible to provide ahighly reliable optical system capable of high-speed access.

EXAMPLE 15

FIGS. 21A-21C show a surface-emission laser diode according to Example15 of the present invention, wherein FIG. 21A shows a top plan view ofthe laser diode, while FIGS. 21B and 21C respectively show the laserdiode in a cross-section taken in a first direction and a seconddirection represented in FIG. 21A.

The laser diode of Example 15 is a surface-emission laser diode havingan active layer of InGaAs and operating in the 0.98 μm band.

The surface-emission laser diode of Example 15 is formed conducting acrystal growth process by using an organic metal chemical vapordeposition (MOCVD) process, wherein trimethyl aluminum (TMA), trimethylgallium (TMG) and trimethyl indium (TMI) are used for the source of thegroup III elements while an arsine (AsH₃) gas is used for the group Vsource. Further, CBr₄ is used for the source of C (carbon) used for thep-type dopant and H₂Se is used for source of Se used as the n-typedopant.

Hereinafter, the fabrication process thereof will be explained accordingto the fabrication process.

More specifically, the device of Example 15 is constructed on an n-GaAssubstrate 1101 and includes an n-GaAs buffer layer 1102 formed on thesubstrate 1101, and an n-AlAs/GaAs lower semiconductor distributed Braggreflector 1103 is formed on the buffer layer 1102, wherein the lowerdistributed Bragg reflector 1103 includes therein repetition of ann-AlAs/GaAs pair repeated for 36 times.

Further, a cavity spacer layer 1104 of undoped GaAs is formed on thelower distributed Bragg reflector 1103, and a multiple quantum wellactive layer 1105 of InGaAs/GaAs is formed further on the cavity spacerlayer 1104.

Further, another cavity spacer layer 1106 of undoped GaAs is formed onthe active layer 1105, and a p-Ga_(0.5)In_(0.5)P layer 1107 and a p-GaAslayer 1108 are grown consecutively.

Here, the semiconductor layers constituting the semiconductordistributed Bragg reflector 1103 are formed with the thickness such thatthere is caused a phase difference of π/2 of laser oscillation light ineach of the semiconductor layers (thickness corresponding to ¼ theoscillation wavelength in the respective semiconductor layers).

Further, the p-GaInP layer 1107 is formed as a part of an upperdistributed Bragg reflector and is grown with the thickness such thatthere occurs a phase difference of laser oscillation light of π/2 in theGaInP layer (¼ thickness of the oscillation wavelength in the GaInPlayer). Further, the thickness of the p-GaAs layer 1108 is set to 50 nm.Thereby, it should be noted that the upper distributed Bragg reflectoris formed by the layers 1107, 1108 and 1109.

It should be noted that the InGaAs/GaAs quantum well active layer 1105and the GaAs cavity spacer layers 1104 and 1106 above and below theactive layer 1105 are formed with respective thicknesses such that thereis caused a total phase difference of 2π in the laser oscillation lightwave in these semiconductor layers. Thereby, there is formed aone-wavelength cavity by these layers. In order to achieve highstimulation emission probability, the InGaAs/GaAs quantum well activelayer 1105 is formed in correspondence to the anti-node of the standingwave of laser oscillation light wave formed at the mid part of thecavity spacer layers 1104 and 1106.

Thus, in the case the refractive index of the cavity spacer layers 1104and 1106 is larger than the refractive index of the semiconductor layers1103 and 1107 constituting a part of the semiconductor distributed Braggreflector as in the case of Example 15, it becomes possible to form thecavity structure (1λ cavity structure) such that the active layerprovided at the mid part of the cavity spacer layers 1104 and 1106 islocated in correspondence to the node of the standing wave by settingthe phase difference caused in the laser oscillation light wave by thecavity spacer layers 1104 and 1106 and the active layer 1105 to be equalto an integer multiple of 2π. In this case, the boundary between thesemiconductor distributed Bragg reflector and the cavity spacer layer1104 or 1106 is formed at the location corresponding to the anti-node ofthe laser oscillation light wave.

Next, a resist pattern is formed on the device surface in correspondenceto the high refractive index region shown in FIG. 21A, and the p-GaAslayer 1108 corresponding to the low refractive index region of FIG. 21Ais removed by a wet etching process while using a sulfuric acid etchant.Here, it should be noted that the wet etching of the p-GaAs layer 1108is conducted selectively by using the underlying p-GaInP layer 1107 asan etching stopper.

Next, removal of the resist is conducted, and after applying anappropriate surface cleaning process, growth of the p-AlAs/GaAssemiconductor distributed Bragg reflector 1109 is conducted by aregrowth process. Here, it should be noted that the growth of thep-AlAs/GaAs semiconductor distributed Bragg reflector 1109 is startedfrom the p-GaAs layer 1108, wherein the p-GaAs layer 1108 is grown withthe thickness chosen such that there is caused a phase difference of π/2therein (¼ thickness of the oscillation wavelength in the GaAs layer).

Thus, in the region where the etching of the p-GaAs layer 1108 isconducted, the respective layers constituting the semiconductordistributed Bragg reflector 1109 are formed to have the thicknesses soas to satisfy the phase condition of multiple reflection of thesemiconductor distributed Bragg reflector with respect to the designedlaser oscillation wavelength.

In the region where the etching removal of the p-GaAs layer 1108 has notbeen made, on the other hand, the GaAs layer is formed to have anincreased thickness as compared with the region where the etching hasbeen made. Further, there is provided a contact layer (not shown) by theoutermost GaAs layer of the upper p-AlAs/GaAs distributed Braggreflector 109 by increasing the carbon concentration at the surface partthereof.

Next, a resist pattern is formed on the device surface, and a currentconfinement structure 1110 is formed in the form of a high resistanceregion by a hydrogen-ion implantation process. Here, the currentconfinement diameter of the current confinement structure 1110 is set to8 μm.

It should be noted that FIG. 21A shows the region not conducted with thehydrogen ion implantation (current injection region) by hatching,wherein it will be noted that the current injection region is formedinside the low refractive index core so as to overlap with the lowrefractive index core spatially.

Next, a p-side electrode 1111 is formed on the device surface and ann-side electrode 1112 is formed on the rear side of the substrate afterpolishing of the rear side. Thereby, a surface-emission laser diodehaving the cross-section shown in FIGS. 21B and 21C is obtained.

Thus, by changing the thickness of the layers located close to thecavity spacer layer and constituting the distributed Bragg reflector byusing the etching and regrowth process, it becomes possible to changethe laser oscillation wavelength in the laser cavity direction (thedirection perpendicular to the substrate) within the plane parallel tothe substrate surface.

Here, it should be noted that the resonant wavelength in the regionwhere the etching has been made is shifted shorter than the resonantwavelength where the etching has not been made.

Here, it should be noted that the effective refractive index in thedirection parallel to the substrate becomes relatively smaller asexplained in relation to the nineteenth mode of the present invention.More specifically, there is achieved a change of effective refractiveindex of about 0.1 for the shift of the resonant wavelength of 50 nm.

Hereinafter, the construction of the surface-emission laser diode ofExample 15 will be explained in detail.

With the surface-emission laser diode of Example 15, there is provided alow refractive index core of a small region having a small effectiverefractive index at the central part of the laser oscillation region inthe direction parallel to the substrate surface as shown in FIG. 21A.

Further, there is provided a cladding layer of high refractive indexregion around the low refractive index core, and with this, there isformed an antiguiding structure.

Further, in the region inside the high refractive cladding layer andcontacting the low refractive index core, there is formed a periodicstructure of refractive index in which a low refractive index region anda high refractive index region are repeated three times.

Here, it should be noted that the low refractive index core has arectangular shape as shown in FIG. 21A, and thus, the low refractiveindex core has a width D1 in the first direction relatively larger thana width D2 thereof in the second direction. More specifically, in thesurface-emission laser diode of Example 15, the low refractive indexcore has the width D1 of 10 μm in the first direction and the width D2of 8 μm in the second direction.

Further, the wavelength in the direction parallel to the substratesurface is determined by the width d and effective refractive index n₀of the low refractive index core and by the effective index n₁ of thehigh refractive index layer surrounding the low refractive index core asshown in Equation (1). Thus, in the first direction and in the seconddirection in which the width d of the low refractive index core isdifferent, the wavelength in the direction parallel to the substratesurface takes different wavelength values.

Herein, the wavelength of the fundamental transverse mode in the highrefractive index cladding layer parallel to the substrate surface isdesignated as λ₁ ¹ in the foregoing first direction and λ₁ ² in theforegoing second direction. Here, it should be noted that the wavelengthin the direction parallel to the substrate surface means the wavelengthcorresponding to the wavenumber of the fundamental transverse mode inthe high refractive cladding layer in the direction parallel to thesubstrate surface.

Further, in both of the first and second directions, the width d1 of thehigh refractive index region and the width d2 of the low refractiveindex region constituting the periodic structure are set, with regard tothe wavelength of the fundamental transverse mode parallel to thesubstrate surface in the first direction, such that there appears aphase difference of odd integer multiple of π/2 in the respectiveregions in the respective directions (odd integer multiple of ¼ thewavelength in the respective media in the direction parallel to thesubstrate surface).

Thus, in the first direction, there is formed a cavity structure similarto the one provided in the vertical direction by using the periodicstructure formed of the regions of different refractive indices as thereflector.

Here, it should be noted that, in the cavity structure formed in thefirst direction, the refractive index of the core is smaller than therefractive index of the high refractive index region provided adjacentthereto in the first direction. Thus, there is formed a half wavelengthcavity structure in which the boundary between the low refractive indexcore and the high refractive index region forms a node of the standingwave.

It should be noted that the wavelength in the low refractive indexregion in the direction parallel to the substrate surface corresponds totwice the width of the low refractive index core.

Thus, by forming the low refractive index region to have the width d2with regard to the width D1 of the low refractive index core in thefirst direction, such that the width d2 is equal to ¼ times the quantity2D1, the foregoing condition is satisfied.

The wavelength λ₁ ¹ in the high refractive index region in the directionparallel to the substrate surface is obtained from Equation (1). Thus,the foregoing condition is satisfied by setting the width d₁ of the highrefractive index to be ¼ times the wavelength λ₁ ¹ parallel to thesubstrate surface in the first direction. In the illustrated example ofthe surface emission laser diode of Example 15, d₁ and d₂ are setrespectively as d₁=1.27 μm (=3λ₁ ¹/4) and d₂=5 μm (=D₁/2).

In this way, the reflection wavelength band (stop band) formed by theperiodic structure is chosen so as to correspond to the wavelength λ₁ ¹Of the fundamental transverse mode in the direction parallel to thesubstrate surface, and thus, the fundamental transverse mode having thepolarization in the first direction is confined into the low refractiveindex core by causing resonance with the cavity structure provided inthe direction parallel to the substrate surface.

Here, the width of the stop band is decreased by decreasing the numberof repetitions in the periodic structure, and this, it is possible toselectively confine the fundamental transverse mode. Thus, the periodicstructure of Example 15 selectively confines the fundamental transversemode having the polarization in the first direction.

In the second direction, on the other hand, the width of the lowrefractive index core is formed to have a reduced width as compared withthe width in the first direction, and thus, the wavelength λ₁ ² of thefundamental transverse mode in the direction parallel to the substratesurface having the polarization in the second direction is offset in theshort wavelength side as compared with the wavelength λ₁ ¹ of thefundamental transverse mode in the direction parallel to the substrateand having the polarization direction in the first direction, while thismeans that the value of the wavelength λ₁ ² is outside the stop bandformed by the periodic structure of the refractive index. As a result,confinement of the transverse mode is weakened in the second directionas compared with the first direction.

Thus, with regard to the transverse mode in the second direction, thereoccurs extensive mode leakage in the direction parallel to the substratesurface, and because of this extensive mode leakage, laser oscillationin the transverse mode having the polarization direction in the seconddirection is effectively suppressed.

Further, it should be noted that the wavelength in the directionparallel to the substrate surface (direction parallel to the substrate)offsets in the shorter wavelength direction with increasing order of thehigher-order transverse mode, and thus, it becomes possible to set thewavelength parallel to the substrate surface in the second direction tobe shorter for all the higher-order transverse modes than the wavelengthof the fundamental transverse mode parallel to the substrate surface inthe first direction, by choosing the width D2 of the low refractiveindex core in the second direction to be smaller than the width D2 ofthe low refractive index core in the first direction. In other words, itis possible to set the wavelength offset from the stop band of theperiodic structure of the refractive index optimized with regard to thefirst direction. Thus, it becomes possible to increase the loss in thesecond direction for all of the higher order transverse modes, and thelaser oscillating which such higher order transverse mode is effectivelysuppressed.

Thus, with the surface emission laser diode of Example 15, it becomespossible to obtain a single fundamental transverse laser oscillationhaving the polarization in the first direction up to high output powerstate.

Further, with the surface-emission laser diode of Example 15, in whichtransverse mode control is not achieved by the selectively oxidizedcurrent confinement layer contrary to the case of the selectivelyoxidized surface-emission laser diode, it becomes possible to increasethe diameter of current confinement. Thereby, the surface-emission laserdiode of Example 15 has the preferable feature of low device resistance.

Further, because there is formed no mesa structure by etching process,the laser diode has excellent heat radiation performance. Thus, thedevice of Example 15 has another advantageous feature that the level ofoutput saturation caused by heat is increased. Further, contrary to thecase of the surface-emission laser diode of the selective oxidationtype, the laser diode of Example 15 does not include a selectivelyoxidized current confinement layer inside the device structure. Thus,parasitic capacitance is decreased and the laser diode can be modulatedat high speed.

While Example 15 has been explained for the case the periodic structureof refractive index provided in the direction perpendicular to the lasercavity direction (direction parallel to the substrate) and the hydrogenion injection region have a rectangular shape in the plan view, thepresent invention is not limited to such a specific construction, and itis also possible to use an elliptic shape as shown in FIG. 22.

In the surface-emission laser diode of FIG. 22, the major axis directionand minor axis direction of ellipse are chosen respectively as the firstdirection and the second direction, and the width of the low refractiveindex region and the high refractive index region of the periodicstructure of refractive index are such that the wavelength of thefundamental transverse mode parallel to the substrate surface in thefirst direction corresponds to the center of the stop band wavelength.

With this, it becomes possible to obtain a single fundamental transversemode oscillation having the polarization in the first directionsimilarly to the device of FIGS. 21A-21C.

EXAMPLE 16

FIGS. 23A-23C show a surface-emission laser diode according to Example16 of the present invention, wherein FIG. 23A shows a top plan view ofthe laser diode, while FIGS. 23B and 23C respectively show the laserdiode in a cross-section taken in a first direction and a seconddirection represented in FIG. 23A.

The laser diode of Example 16 is a surface-emission laser diode havingan active layer of GaInNAs and operating in the 1.3 μm band. The surfaceemission laser diode of Example 16 is formed according to the processand steps similar to those used with Example 15 while using dimethylhydrazine (DMHy) as the nitrogen source of the active layer.

Hereinafter, the structure thereof will be explained.

More specifically, the device of Example 16 is constructed on an n-GaAssubstrate 1201 and includes an n-GaAs buffer layer 1202 formed on thesubstrate 1201, and an n-AlAs/GaAs lower semiconductor distributed Braggreflector 1203 is formed on the buffer layer 1202, wherein the lowerdistributed Bragg reflector 1203 includes therein repetition of ann-AlAs/GaAs pair repeated for 36 times.

Further, a cavity spacer layer 204 of undoped GaAs is formed on thelower distributed Bragg reflector 1203, and a multiple quantum wellactive layer 1205 of GaInNAs/GaAs multiple quantum well structure isformed further on the cavity spacer layer 1204.

Further, another cavity spacer layer 1206 of undoped GaAs is formed onthe active layer 1205, and a p-Ga_(0.5)In_(0.5)P layer 1207 and a p-GaAslayer 1208 are grown consecutively.

Here, the semiconductor layers constituting the semiconductordistributed Bragg reflector 1203 are formed with the thickness such thatthere is caused a phase difference of π/2 of laser oscillation lightwave in each of the semiconductor layers (thickness corresponding to ¼the oscillation wavelength in the respective semiconductor layers),similarly to the case of Example 15. Thereby, there is formed a cavityhaving the 1λ cavity structure similarly to Example 15.

Further, the p-Ga_(0.5)In_(0.5)P layer 1207 is formed to have athickness such that there appears a phase difference of π/2 in the laseroscillation light in the Ga_(0.5)In_(0.5)P layer (¼ thickness of theoscillation wavelength in the respective semiconductor layers). Further,the thickness of the p-GaAs layer is set to 50 nm.

Next, by using a photolithographic process, there is formed a resistpattern on the device surface in correspondence to the high refractiveindex region shown in FIG. 23A, and the GaAs layer 208 is removed byusing a sulfuric acid etchant.

Next, the resist is removed, and after appropriate surface cleaningprocessing, growth of the layer 1209 forming a part of the upperp-AlAs/GaAs distributed Bragg reflector is conducted such that thegrowth starts from the GaAs layer 1208. In the surface-emission laserdiode, it should be noted that the upper distributed semiconductor Braggreflector is formed of the layers 1207, 1208 and 1209. Thereby, itshould be noted that the first GaAs layer 1208 forming the firstregrowth layer is grown so as to have a thickness in which a phasedifference of π/2 is caused for the oscillation light wave in the GaAslayer 1208 (¼ thickness of oscillation wavelength in the GaAs layer1208).

Thus, in the region of the p-GaAs layer 1208 where the etching has beenmade, the layers constituting the semiconductor distributed Braggreflector are formed with respective thicknesses that satisfy the phasecondition of multiple reflection of the semiconductor distributed Braggreflector. On the other hand, in the region of the p-GaAs layer 1208where the etching has not been made, the GaAs layer 208 is formed withincreased thickness as compared with the region where the etching hasbeen made.

Next, a resist pattern is formed on the device surface, and a currentconfinement structure 210 is formed in the form of a high resistanceregion by a hydrogen-ion implantation process. Here, the currentconfinement diameter of the current confinement structure 1210 is set to14 μm.

With the surface-emission laser diode of Example 16, the low refractiveindex core has a width D1 of 14 μm in the first direction relatively andthe width D2 of 20 μm in the second direction.

It should be noted that the region not conducted with the hydrogen ionimplantation (current injection region) is shown in FIG. 23A byhatching, wherein it will be noted that the current injection region isformed inside the low refractive index core so as to overlap with thelow refractive index core spatially.

Here, it should be noted that the resonant wavelength in the regionwhere the etching of the p-GaAs layer 1208 has been made is shiftedrelatively shorter than the resonant wavelength where the etching hasnot been made. Thereby, there is formed a low refractive index regionhaving a small effective refractive index in the direction perpendicularto the laser cavity direction (direction parallel to the substrate).

With the surface-emission laser diode of Example 16, there is provided alow refractive index core of a small region having a small effectiverefractive index at the central part of the laser oscillation region inthe direction parallel to the substrate surface as shown in FIGS.23A-23C.

Further, there is provided a cladding layer of high refractive indexregion around the low refractive index core, and with this, there isformed an antiguiding structure.

Further, in the region inside the high refractive cladding layer andcontacting with the low refractive index core, there is formed aperiodic structure of refractive index in the first direction in which alow refractive index region and a high refractive index region arerepeated three times. with the device of Example 16, there is providedno periodic structure in the second direction as shown in FIGS. 23A-23C.

Further, in the first direction, the width d1 of the high refractiveindex region and the width d2 of the low refractive index regionconstituting the periodic structure are set, with regard to thewavelength of the fundamental transverse mode parallel to the substratesurface in the first direction, such that there appears a phasedifference of odd integer multiple of π/2 in the respective regions inthe respective directions (odd integer multiple of ¼ the wavelength inthe respective media in the direction parallel to the substratesurface).

Thus, in the first direction, there is formed a cavity structure similarto the one provided in the vertical direction by using the periodicstructure formed of the regions of different refractive indices as thereflector similarly to Example 15.

Thus, the fundamental transverse mode having polarization in the firstdirection cause resonance with the foregoing periodic structure and isconfined in the direction parallel to the substrate. Thereby, loss bymode leakage in the direction parallel to the substrate surface iseffectively suppressed.

With regard to the second region, on the other hand, only the highrefractive index cladding makes contact with the low refractive indexcore, and there is formed a complete half waveguide structure by thereal refractive index difference. Thus, there is caused mode leakage forthe entire modes in the direction parallel to the substrate surface, andthus, there appears large mode loss in the polarization in the seconddirection.

Here, it should be noted that the wavelength in the low refractive indexregion in the direction parallel to the substrate surface corresponds totwice the width of the low refractive index core as explained withreference to Example 15.

Thus, by forming the low refractive index region to have the width d2with regard to the width D1 of the low refractive index core in thefirst direction, such that the width d2 is equal to ¼ times the quantity2D1, the foregoing condition is satisfied.

The wavelength λ₁ ¹ in the high refractive index region in the directionparallel to the substrate surface is obtained from Equation (1). Thus,the foregoing condition is satisfied by setting the width d₁ of the highrefractive index to be ¼ times the wavelength λ₁ ¹ parallel to thesubstrate surface in the first direction. Here, λ₁ ¹ represents thewavelength of the fundamental transverse mode in the high refractiveindex cladding corresponding to the wavenumbers in the directionparallel to the substrate surface. In the surface emission laser diodeof Example 16, d₁ and d₂ are set respectively as d₁=1.69 μm (=3λ₁ ¹/4)and d₂=7 μm (=D₁/2).

From the foregoing, the loss of the fundamental transverse mode havingthe polarization in the first direction becomes minimum with thesurface-emission laser diode of Example 16, and there occurs laseroscillation selectively in the fundamental transverse mode withpolarization in the first direction. Further, with the periodicstructure of refractive index provided in the first direction, thesurface-emission laser diode of Example 16 can maintain the single modefundamental transverse laser oscillation up to high output power state.

Thus, with the surface emission laser diode of Example 16, it becomespossible to obtain a single fundamental transverse laser oscillationhaving the polarization in the first direction up to high output powerstate.

Further, with the surface-emission laser diode of Example 15, in whichtransverse mode control is not achieved by the selectively oxidizedcurrent confinement layer contrary to the case of the selectivelyoxidized surface-emission laser diode, it becomes possible to increasethe diameter of current confinement. Thereby, the surface-emission laserdiode of Example 15 has the preferable feature of low device resistance.

Further, because there is formed no mesa structure by etching process,the laser diode has excellent heat radiation performance. Thus, thedevice of Example 15 has another advantageous feature that the level ofoutput saturation caused by heat is increased. Further, contrary to thecase of the surface-emission laser diode of the selective oxidationtype, the laser diode of Example 15 does not include a selectivelyoxidized current confinement layer inside the device structure. Thus,parasitic capacitance is decreased and the laser diode can be modulatedat high speed.

While the foregoing explanation has been made for the case a rectangularlow refractive index core shown in FIGS. 23A-23C is provided as anexample of providing the confinement structure of the transverse modeonly in the first direction, the present invention is not limited tosuch a specific construction, and it is also possible to use a lowrefractive index core of elliptic shape as shown in FIG. 24.

FIG. 24 shows an example of the surface-emission laser diode having aconstruction similar to the one shown in FIGS. 23A-23C except that a lowrefractive index core of elliptical shape is used.

In FIG. 24, it should be noted that the layered structure in thevertical direction, the resonance condition by the periodic structure ofrefractive index in the direction parallel to the substrate source, orthe like, are determined identical to the case of the device of FIGS.23A-23C, except that the core of the low refractive index and theperiodic structure of refractive index are formed to have an ellipticalshape.

In the surface-emission laser diode of FIG. 24, it should be noted thatthe periodic structure of refractive index is provided so as to extendover an angular range θ including the first direction. In this way, inthe case the core is formed to have an elliptical shape or circularshape, it is possible to secure a certain angular range with regard tothe first direction.

EXAMPLE 17

FIGS. 25A-25C show a surface-emission laser diode according to Example17 of the present invention, wherein FIG. 25A shows a top plan view ofthe laser diode, while FIGS. 25B and 25C respectively show the laserdiode in a cross-section taken in a first direction and a seconddirection represented in FIG. 25A.

The laser diode of Example 17 is a surface-emission laser diode havingan active layer of GaAs and operating in the 0.85 μm band.

The surface-emission laser diode of Example 17 is formed conducting acrystal growth process by using an organic metal chemical vapordeposition (MOCVD) process similarly to the case of Example 15 andExample 16

Hereinafter, the fabrication process thereof will be explained accordingto the fabrication process.

More specifically, the device of Example 17 is constructed on an n-GaAssubstrate 1301 and includes an n-GaAs buffer layer 1302 formed on thesubstrate 1301, and an n-AlAs/GaAs lower semiconductor distributed Braggreflector 1303 is formed on the buffer layer 1302, wherein the lowerdistributed Bragg reflector 1303 includes therein repetition of ann-AlAs/Al_(0.15)Ga_(0.85)As pair repeated for 40 times.

Further, a cavity spacer layer 1304 of undoped Al_(0.15)Ga_(0.85)As isformed on the lower distributed Bragg reflector 1303, and a multiplequantum well active layer 1305 of GaAs/Al_(0.15)Ga_(0.85)As is formedfurther on the cavity spacer layer 1304.

Further, another cavity spacer layer 1306 of undopedAl_(0.15)Ga_(0.85)As is formed on the active layer 305, and ap-Ga_(0.5)In_(0.5)P layer 1307 and a p-Al_(0.15)Ga_(0.85)As layer 308are grown consecutively.

Here, the semiconductor layers constituting the semiconductordistributed Bragg reflector 1303 are formed with the thickness such thatthere is caused a phase difference of π/2 of laser oscillation light ineach of the semiconductor layers (thickness corresponding to ¼ theoscillation wavelength in the respective semiconductor layers).

Further, the p-Ga_(0.5)In_(0.5)P layer 1307 is formed as a part of anupper distributed Bragg reflector and is grown with the thickness suchthat there occurs a phase difference π/2 in the laser oscillation lightwave in the Ga_(0.5)In_(0.5)P layer (¼ thickness of the oscillationwavelength in the Ga_(0.5)In_(0.5)P layer). Further, the thickness ofthe p-Al_(0.15)Ga_(0.85)As layer 1308 is set to 30 nm.

Further, the GaAs/Al_(0.15)Ga_(0.85)As quantum well active layer 1305and the GaAs cavity spacer layers 1304 and 1306 above and below theactive layer 1305 are formed with respective thicknesses such that thereis caused a total phase difference of 2π in the laser oscillation lightwave in these semiconductor layers. Thereby, there is formed aone-wavelength cavity by these layers. In order to achieve highstimulation emission probability, the GaAs/Al_(0.15)Ga_(0.85)As quantumwell active layer 1305 is formed in correspondence to the anti-node ofthe standing wave of laser oscillation light wave formed at the mid partof the cavity spacer layers 1304 and 1306.

Next, a resist pattern is formed on the device surface in correspondenceto the high refractive index region shown in FIG. 25A, and thep-Al_(0.15)Ga_(0.85)As layer 308 corresponding to the low refractiveindex region of FIG. 25A is removed by a wet etching process while usinga sulfuric acid etchant. Here, it should be noted that the wet etchingof the p-GaAs layer 1308 is conducted selectively by using theunderlying p-Al_(0.15)Ga_(0.85)InP layer 307 as an etching stopper.

Next, removal of the resist is conducted, and after applying anappropriate surface cleaning process, growth of thep-AlAs/Al_(0.15)Ga_(0.85)As semiconductor distributed Bragg reflector1309 is conducted by a regrowth process for twenty six periods. Here, itshould be noted that the growth of the p-AlAs/Al_(0.15)Ga_(0.85)Assemiconductor distributed Bragg reflector 1309 is started from thep-Al_(0.15)Ga_(0.85)As layer, wherein the p-Al_(0.15)Ga_(0.85)As layeris grown with the thickness chosen such that there is caused a phasedifference of π/2 therein (¼ thickness of the oscillation wavelength inthe Al_(0.15)Ga_(0.85)As layer).

Further, there is provided a contact layer (not shown) by the outermostGaAs layer of the upper p-AlAs/Al_(0.15)Ga_(0.85)As distributed Braggreflector 1309 by increasing the carbon doping at the surface partthereof.

Thus, in the region where the etching of the p-Al_(0.15)Ga_(0.85)Aslayer 1308 is conducted, the respective layers constituting thesemiconductor distributed Bragg reflector 1309 are formed to have thethicknesses so as to satisfy the phase condition of multiple reflectionof the semiconductor distributed Bragg reflector with respect to thedesigned laser oscillation wavelength.

In the region where the etching removal of the p-Al_(0.15)Ga_(0.85)Aslayer 1308 has not been made, on the other hand, theAl_(0.15)Ga_(0.85)As layer is formed to have an increased thickness ascompared with the region where the etching has been made.

Next, a resist pattern is formed on the device surface, and a currentconfinement structure 1310 is formed in the p-AlAs/Al_(0.15)Ga_(0.85)Assemiconductor distributed Bragg reflector 1309 in the form of a highresistance region by a hydrogen-ion implantation process. Here, thecurrent confinement diameter of the current confinement structure 1310is set to 8 μm.

It should be noted that FIG. 25A shows the region not conducted with thehydrogen ion implantation (current injection region) by hatching,wherein it will be noted that the current injection region is formedinside the low refractive index core so as to overlap with the lowrefractive index core spatially.

Next, a p-side electrode 1311 is formed on the device surface and ann-side electrode 1312 is formed on the rear side of the substrate afterpolishing of the rear side. Thereby, a surface-emission laser diodeshown in FIGS. 25A-25C is obtained.

Hereinafter, the construction of the surface-emission laser diode ofExample 17 will be explained in detail.

With the surface-emission laser diode of Example 17, there is provided alow refractive index core of a small region having a small effectiverefractive index at the central part of the laser oscillation region inthe direction parallel to the substrate surface as shown in FIGS.25A-25C.

Further, there is provided a cladding layer of high refractive indexregion around the low refractive index core, and with this, there isformed an antiguiding structure.

Further, in the region inside the high refractive cladding layer andcontacting with the low refractive index core, there is formed aperiodic structure of refractive index in which a low refractive indexregion and a high refractive index region are repeated three times.

In Example 17, it should be noted that the low refractive index core hasa square shape as shown in FIG. 25A, and thus, the low refractive indexcore has a width D1 of 8 μm.

In Example 17, it should be noted that the construction of therefractive index is different between the fist direction and the seconddirection shown in FIG. 25A, wherein the width d1 and d2 of the highrefractive index region and the low refractive index region are set inthe first direction such that there appears a phase difference of an oddinteger multiple of π/2 in the respective regions (odd integer multipleof ¼ of the wavelength λ₁ ¹ in the direction parallel to the substratesurface in the respective media). Here, it should be noted that λ₁ ¹represents the wavelength of the fundamental transverse mode in the highrefractive index cladding layer in the direction parallel to thesubstrate taken in the first and second direction, similarly to Example15. (In Example 17, in which the low refractive index core has a squareshape, the wavelength λ₁ ¹ in the first direction and λ₁ ² in the seconddirection are identical). More specifically, with the surface-emissionlaser diode of Example 17, widths d₁ and d₂ are set to d₁=1.1 μm(=3λ₁¹/4), d₂=5 μm (=D₁/2)

Thus, in the first direction, there is formed a cavity structure similarto the one formed in the vertical direction by using the periodicstructure formed of regions of different refractive indices as themirror.

In the second direction, on the other hand, the widths d3 and d4 of thehigh refractive index region and the low refractive index region areprovided relatively larger than the widths d₁ and d₂. More specifically,d₃ and d₄ are set to be about 1.3 times larger than d₁ and d₂ as d₃=1.43μm, d₄=5.2 μm.

In this way, by setting the width of the low refractive index region andthe width of the high refractive index region in the second direction tobe larger than the width of the low refractive index region and thewidth of the high refractive index determined so as to achieve optimumconfinement of the fundamental transverse mode in the first direction,it becomes possible to decrease the reflectivity for all of the modes.

As explained with reference to Equation (1), the wavelength of thehigher-order transverse mode is always offset in the short wavelengthdirection as compared with the wavelength of the fundamental transversemode, and thus, by setting the stop band wavelength of the periodicstructure in the long wavelength side, it becomes possible to set thereflectivity for all of the modes.

Thus, with the device of Example 17, the mode having electric fieldamplitude (polarization) only in the first direction is selectivelyconfined, while there occurs only weak confinement in the seconddirection for all of the modes. Thus, laser oscillation is suppressedeffectively in those modes having polarization in the second directiondue to the mode leakage and associated loss, and laser oscillationoccurs selectively in the fundamental transverse mode havingpolarization in the first direction.

Thus, with the surface emission laser diode of Example 17, it ispossible to obtain a single fundamental mode laser oscillation havingpolarization aligned in the first direction, up to high out put powerlevel.

Further, with the surface-emission laser diode of Example 17, in whichtransverse mode control is not achieved by the selectively oxidizedcurrent confinement layer contrary to the case of the selectivelyoxidized surface-emission laser diode, it becomes possible to increasethe diameter of current confinement. Thereby, the surface-emission laserdiode of Example 17 has the preferable feature of low device resistance.

Further, because there is formed no mesa structure by etching process,the laser diode has excellent heat radiation performance. Thus, thedevice of Example 17 has another advantageous feature that the level ofoutput saturation caused by heat is increased. Further, contrary to thecase of the surface-emission laser diode of the selective oxidationtype, the laser diode of Example 17 does not include a selectivelyoxidized current confinement layer inside the device structure. Thus,parasitic capacitance is decreased and the laser diode can be modulatedat high speed.

While Example 17 has been explained for the case the periodic structureof refractive index provided in the direction perpendicular to the lasercavity direction (direction parallel to the substrate) and the hydrogenion injection region have a rectangular shape in the plan view, thepresent invention is not limited to such a specific construction, and itis also possible to use an elliptic shape as shown in FIG. 22.

While explanation has been made for various examples above with regardto the case of using an MOCVD process for the crystal growth process, itis also possible with the present invention to use a molecular beamcrystal growth (MBE) process or other crystal growth processes. Further,it is possible to use a semi-insulating substrate or a p-type substratein place of the n-type substrate. Further, the laser oscillationwavelength is not limited to the foregoing 0.85 μm band or 1.3 μm band,and the laser diode can oscillate with the wavelength of 0.65 μm band,0.98 μm band, 1.5 μm band, or the like.

According to the oscillation wavelength, it is possible to use othermaterials for the laser diode. For example, it is possible to use anAlGaInP mixed crystal for the 0.65 μm band. For the 0.98 μm band, it ispossible to use an InGaAs mixed crystal. Further, a GaInNAs(Sb) mixedcrystal can be used for the 1.5 μm band. Thereby, materials transparentin such a wavelength band are used for the distributed Bragg reflectorwith such a combination so as to maximize the refractive indexdifference therebetween.

Further, the device structure may be different from the structureexplained with reference to the foregoing examples. Further, the devicesexplained with the foregoing examples may be tuned to oscillate withother oscillation wavelengths. By choosing the materials andconstruction of the distributed Bragg reflector appropriately accordingto the desired oscillation wavelength, any of the devices explainedabove can be tuned to the desired oscillation wavelength.

In order to reduce the device resistance further, it is effective toprovide a heterospike buffering layer between the Al(Ga)As/GaAsheterointerface with a composition intermediate therebetween. Further,such a heterospike buffering layer may be provided to the interface ofthe selective oxidizing layer.

With regard to the heterospike buffering layer, it is possible to use asingle layer having a composition intermediate of the two layersconstituting the heterointerface or combination of plural layers ofdifferent compositions. Further, it is also possible to change thecomposition continuously.

EXAMPLE 18

FIG. 26 shows a surface-emission laser array according to Example 18 ofthe present invention. More specifically, FIG. 18 shows the constructionof a monolithic laser array in which the surface-emission laser diodesof the present invention are arranged two-dimensionally in the 4×4formation in a top plan view.

In FIG. 26, there is provided wirings on the upper electrodesindividually so as to drive the laser diodes independently. Thesurface-emission laser diode array of FIG. 26 is formed by the processand method similar to the one used in any of the examples explainedbefore.

For the surface-emission laser diodes constituting the surface-emissionlaser array of FIG. 26, the surface-emission laser diode of any ofExamples 15 through Example 17 (the surface-emission laser diode inwhich the polarization direction is aligned in a specific desireddirection and capable of maintaining single fundamental transverse modelaser oscillation up to high output power state) is used. Thereby, thepolarization direction is controlled by patterning conducted byphotolithographic process, it is possible to set the polarizationdirection individually for each of the laser diode elements constitutingthe array.

Thus, with Example 18, it becomes possible to obtain a surface-emissionlaser array in which the polarization direction is controlled stably ina desired specific direction and is capable of oscillating in singletransverse mode up to high output state.

EXAMPLE 19

FIG. 27 is a diagram showing a surface-emission laser module as anexample of the optical interconnection system according to Example 19,while FIG. 28 is a diagram showing an example of a parallel opticalinterconnection system that connects different devices.

The laser array module of FIG. 27 is constructed by mounting aone-dimensional monolithic surface-emission laser array of the presentinvention, a microlens array and a fiber array on a silicon substrate.

Here, the surface-emission laser array is provided in the directiontoward the fibers and are coupled with the silica single mode fibersmounted on V-shaped grooves formed on the silicon substrate via themicrolens array. The surface-emission laser array has an oscillationwavelength of 1.3 μm band and achieves high speed transmission over thesilica single mode fibers.

With the interconnection system of FIG. 28, a device land a device 2 areconnected by an optical fiber array. Thereby, the device 1 at thetransmission side includes a one-dimensional laser array module thatuses the surface-emission laser diode or the surface-emission laserarray of the present invention and a driving circuit thereof. Further,the device 2 at the reception side includes a photodiode array moduleand a signal detection circuit.

With the optical interconnection system of Example 19, a stablefundamental transverse mode laser oscillation is obtained with regard tothe environmental temperature change or change of driving conditionwhile maintaining the polarization in a specific direction, as a resultof use of the surface-emission laser array of the present invention.Because there occurs little change of coupling efficiency with theoptical fibers, it becomes possible with the present invention toconstruct a highly reliable interconnection system.

While foregoing example has been explained with regard to the paralleloptical interconnection system, it is also possible to construct aserial transmission system by using a single device. Further, it is alsopossible to apply the system of Example 19 to inter board connection,inter chip connection and intra chip connection.

EXAMPLE 20

FIG. 29 shows an optical communicating system according to Example 20for the case the optical communication system is used as an optical LAN.

With the optical LAN system of FIG. 29, the surface-emission laser diodeof surface-emission laser array of the present invention is used for theoptical source of optical transmission between a server and a coreswitch, between a core switch and individual switches, and betweenindividual switches and individual terminals.

Thereby, connection between the devices is achieved by using a silicasingle mode fiber or a multimode fiber.

By using the surface-emission laser diode or surface-emission laserarray of the present invention for the optical source, in which thesurface-emission laser diode or surface-emission laser array of thepresent invention is used for the optical source, stable fundamentaltransverse mode laser oscillation is achieved with the LAN system ofFIG. 29 while maintaining the polarization is a predetermined direction,in spite of environmental temperature change or change of drivecondition, and it became possible to construct a highly reliable systembecause of little change of coupling efficiency with optical fibers.

EXAMPLE 21

FIG. 30 is a diagram showing an elector-photographic system according toExample 21 of the present invention.

The electrophotographic system of FIG. 30 comprises a photosensitivedrum, scanning and converging optical system (optical scanning system),a writing optical source and a synchronization control part, wherein thesurface-emission laser diode or the surface-emission laser array of thepresent invention is used for the wiring optical source.

The electrophotographic system of FIG. 30 is controlled by asynchronization control circuit, and the optical beam from the opticalsource is focused upon the photosensitive drum by the scanning andconverging optical system that includes a polygonal mirror. Thereby,there is formed an electrostatic latent image on the photosensitivedrum. For the surface-emission laser diode, it is possible to use theone having the oscillation wavelength of 780 nm band.

Conventionally, it has been difficult with the surface-emission laserdiode to operate with high output power because of extensive heatgeneration, while in the case of the surface-emission laser diode of thepresent invention, operation with higher output power becomes possibleas compared with the conventional device. Thus, the surface-emissionlaser diode of the present invention is suitable for the writing opticalsource of the electrophotographic system.

Further, because the laser oscillation is in the single fundamentaltransverse mode, a single-peak far field image is obtained, and focusingof the optical beam is achieved easily. With this, a high definitionimage is obtained with the present invention.

Further, with the red surface-emission laser diode that uses the AlGaInPmaterial for the active layer, laser oscillation at about 650 nm ispossible, while this oscillation wavelength is shorter than the case ofusing an AlGaAs material. Thereby, the tolerance of design in theoptical system is increased. Thus, such a red surface-emission laserdiode is suitable for the writing optical source of high definitionelectrophotographic systems.

It should be noted that such a surface-emission laser diode can beconstructed by using the material of AlGaInP system for the active layerand by using the material of AlGaAs or AlGaInP system for thedistributed Bragg reflector.

Further, it is possible to achieve crystal growth in lattice matchingwith the GaAs substrate with the use of such a material, and thus, it ispossible to use such an AlAs material for the selective oxidation layer.

On the other hand, with the use of AlGaInP material, there arises aproblem that the laser diode becomes extremely susceptible totemperature change, and associated with this, problems such assaturation of output power or failure of laser oscillation are causedwith temperature increase associated with device heat generation.

With the surface-emission laser diode fabricated with the presentinvention, the higher-order transverse mode distribution is shiftertoward the mesa sidewall surface as a result of use of the antiguidingstructure, while such a shift decreases the degree of coupling of thehigher-order transverse mode distribution with the gain region. Thereby,laser oscillation with higher-order transverse mode is effectivelysuppressed, while this enables use of larger diameter for the currentconfinement region.

Thus, with the present invention, it becomes possible to decrease thediameter of the current confinement region, which has been a problemwith a red surface-emission laser diode, and it becomes possible torealize a device of low resistance.

Thus, with the present invention, it becomes possible to realize a redsurface-emission laser diode of reduced heat generation and capable ofoscillating in the single fundamental transverse mode with higher outputpower as compared with the conventional device, while such a laser diodeis quite suitable for the writing optical source of electrophotographicsystem.

Further, with the surface-emission laser diode of the present invention,the polarization is controlled in a specific direction, and because ofthis, uniform beam spot is obtained on the photosensitive drum, andthus, properties preferable for the optical source of anelectrophotographic system is provided.

Thus, while there exists a problem that the beam spot shape may changedepending on the polarization direction, the surface-emission laserdiode of the present invention can avoid this problem because thepolarization direction is controlled with the present invention. Thus,the surface-emission laser diode constructed by the present invention issuitable for the writing optical source of an electrophotographicsystem.

Further, as a result of use of the surface-emission laser array ofExample 18, it becomes possible to increase the writing speed ascompared with the conventional device. Thereby, it becomes possible withthe present invention to obtain a high speed and high definitionelectrophotographic system.

EXAMPLE 22

FIG. 31 is a diagram showing an optical disk system according to Example22 of the present invention.

The optical disk system of FIG. 31 is formed of an optical disk, anoptical system 1, an optical system 2, a beam splitter, an opticaldetector, a laser optical source, and a synchronization part(synchronization control circuit), wherein the surface-emission laserdiode or surface-emission laser array of the present invention is usedfor the laser optical source.

Here, it should be noted that the optical system 1, optical system 2,beam splitter, optical detector and the laser optical source constitutean optical head, wherein the optical head is driven by an actuator andachieves access to an arbitrary track on the disk. Here, the opticalsystem 1 is constructed by a diffraction grating and a beam expansionlens, while the optical system 2 is constructed of a ¼ wavelength plateand a beam converging lens.

With the optical disk system of FIG. 20, the laser beam from the lasersource is focused upon the disk surface by the optical source 1 and theoptical source 2 under control of the synchronization circuit, and thedisk surface is irradiated with the laser beam.

On the disk surface, there is formed a track by information pitsarranged in the form of a regular array, and the laser beam reflectedfrom such a disk surface is directed to the optical detector in thereading operation by the beam splitter after passing through the opticalsystem 2 again. In the optical detector, the information signal and alsothe tracking signal formed by the information pits are detected, andservo control of the optical head is achieved based on the detectedsignals with regard to the distance between the disk and the head andwith regard to the head and the track.

With the surface-emission laser diode of the present invention, thepolarization direction is aligned in a specific direction and a largeroutput power is possible as compared with the conventional device whilemaintaining the single fundamental transverse mode operation. Because ofthis, a single peak beam spot is obtained stably. Thereby, the opticalsystem necessary for beam shaping is simplified, and the cost of theoptical disk system is reduced.

Because the polarization direction is controlled easily in a specificdirection, the surface-emission laser diode of the present invention isparticularly useful for the reading optical source of a magneto-opticaldisk.

Because it becomes possible to obtain a single peak beam spot withreliability, the optical disk system constructed by using thesurface-emission laser diode of the present invention provides excellentreliability. Further, it becomes possible to achieve high speed readingas a result of use of the surface-emission laser array of Example 10.

Thus, with the present invention, it becomes possible to obtain a highspeed and highly reliable optical disk system.

Further, the present invention is by no means limited to the embodimentsdescribed heretofore, but various variations and modifications may bemade without departing from the scope of the invention.

The present invention is based on Japanese priority applications2003-396815 filed on Nov. 27, 2003, 2004-148902 filed on May 19, 2004,2004-184492 filed on Jun. 23, 2004, and 2004-322041 filed on Nov. 5,2004, the entire contents of which are incorporated herein as reference.

1. A surface-emission laser diode, comprising: an active layer; a pairof cavity spacer layers formed at both sides of said active layer; acurrent confinement structure defining a current injection region intosaid active layer; and a pair of distributed Bragg reflectors opposingwith each other across a structure formed of said active layer and saidcavity spacer layers, said current confinement structure being formed bya selective oxidation process of a semiconductor layer, said pair ofdistributed Bragg reflectors being formed of semiconductor materials,wherein there is provided a region containing an oxide of Al and havinga relatively low refractive index as compared with a surrounding regionin any of said semiconductor distributed Bragg reflector or said cavityspacer layer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 2. Asurface-emission laser diode according to claim 1, wherein there isprovided any of a GaAs layer or a GaInP mixed crystal layer adjacentwith an AlGaAs mixed crystal layer constituting said region ofrelatively low refractive index with respect to said surrounding regionnot injected with said molecules containing oxygen and located on anidentical plane of said part where said ion implantation of saidmolecules has been made.
 3. A surface emission laser diode according toclaim 1, wherein said AlGaAs mixed crystal layer, constituting saidregion of relatively low refractive index with respect to saidsurrounding region not made with ion implantation of moleculescontaining oxygen and located on said identical plane perpendicular tosaid laser cavity direction, is provided at a location corresponding toan anti-node of a standing wave of laser oscillation occurring in saidcavity structure, said AlGaAs mixed crystal layer being doped to ahigher concentration level as compared with other AlGaAs mixed crystallayers constituting said surface-emission laser diode.
 4. Asurface-emission laser diode according to claim 1, wherein said regionof relatively low refractive index is provided in plural numbers.
 5. Asurface-emission laser diode according to claim 1, wherein said regionof relatively low refractive index is provided inside an n-typedistributed Bragg reflector constituting one of said pair of distributedBragg reflectors.
 6. A surface-emission laser diode according to claim1, wherein there is further provided a region of relatively highrefractive index around said region of relatively low refractive indexprovided in spatial overlapping with said laser cavity region in saidlaser cavity direction, and wherein there is further provided a claddingregion of low refractive index with respect to said high refractiveregion such that said cladding region is located around said region ofhigh refractive index.
 7. A surface-emission laser diode according toclaim 6, wherein there is provided an anisotropy in a width of said highrefractive region surrounded by said cladding region.
 8. Asurface-emission laser diode according to claim 6, wherein said claddingregion is provided in a pair in a direction perpendicular to said lasercavity direction across a laser cavity region.
 9. A surface-emissionlaser diode according to claim 1, wherein said region of relatively lowrefractive index has an anisotropic shape.
 10. A surface-emission laserdiode according to claim 1, wherein said active layer comprises a groupIII-V compound semiconductor material, said group III element comprisesat least one of Ga and In, and wherein said group V element comprisesone or more of As, N, Sb and P.
 11. A surface-emission laser diode,comprising: an active layer; a pair of cavity spacer layers formed atboth sides of said active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across astructure formed of said active layer and said pair of cavity spacerlayers, said current confinement structure comprising a high resistanceregion formed by an ion implantation process, said pair of distributedBragg reflectors being formed of semiconductor materials, wherein thereis provided a region containing an oxide of Al and having a relativelylow refractive index than a surrounding region in any of saidsemiconductor distributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 12. A surface-emissionlaser diode according to claim 11, wherein there is provided any of aGaAs layer or a GaInP mixed crystal layer adjacent with an AlGaAs mixedcrystal layer constituting said region of relatively low refractiveindex with respect to said surrounding region not injected with saidmolecules containing oxygen and located on an identical plane of saidpart where said ion implantation of said molecules has been made.
 13. Asurface emission laser diode according to claim 11, wherein an AlGaAsmixed crystal layer, constituting said region of relatively lowrefractive index with respect to said surrounding region not made withion implantation of molecules containing oxygen and located on anidentical plane perpendicular to said laser cavity direction, isprovided at a location corresponding to an anti-node of a standing waveof laser oscillation occurring in said cavity structure, said AlGaAsmixed crystal layer being doped to a higher concentration level ascompared with other AlGaAs mixed crystal layers constituting saidsurface-emission laser diode.
 14. A surface-emission laser diodeaccording to claim 11, wherein said region of relatively low refractiveindex is provided in plural numbers.
 15. A surface-emission laser diodeaccording to claim 11, wherein said region of relatively low refractiveindex is provided inside an n-type distributed Bragg reflectorconstituting one of said pair of distributed Bragg reflectors.
 16. Asurface-emission laser diode according to claim 11, wherein there isfurther provided a region of relatively high refractive index aroundsaid region of relatively low refractive index provided in spatialoverlapping with said laser cavity region in said laser cavitydirection, and wherein there is further provided a cladding region oflow refractive index with respect to said high refractive region suchthat said cladding region is located around said region of highrefractive index.
 17. A surface-emission laser diode according to claim16, wherein there is provided an anisotropy in a width of said highrefractive region surrounded by said cladding region.
 18. Asurface-emission laser diode according to claim 16, wherein saidcladding region is provided in a pair in a direction perpendicular tosaid laser cavity direction across a laser cavity region.
 19. Asurface-emission laser diode according to claim 11, wherein said regionof relatively low refractive index has an anisotropic shape.
 20. Asurface-emission laser diode according to claim 11, wherein said activelayer comprises a group III-V compound semiconductor material, saidgroup III element comprises at least one of Ga and In, and wherein saidgroup V element comprises one or more of As, N, Sb and P.
 21. Asurface-emission laser diode, comprising: an active layer; a currentconfinement structure defining a current injection region into saidactive layer; and a pair of distributed Bragg reflectors opposing witheach other across said active layer; said current confinement structurebeing formed of a selective oxidation process of a semiconductormaterial, said surface-emission laser diode comprising an AlGaAs mixedcrystal layer, a part of said AlGaAs mixed crystal layer having spatialoverlapping with said current injection region in a laser cavitydirection, said part of said AlGaAs mixed crystal being formed byselective ion implantation of molecules containing oxygen and a thermalannealing process following said selective ion implantation process,said part of said AlGaAs mixed crystal having a relatively lowrefractive index as compared with a surrounding region wherein ionimplantation of molecules containing oxygen is not made and located onan identical plane of said part, said plane being perpendicular to saidlaser cavity direction.
 22. A surface-emission laser diode according toclaim 21, wherein there is provided any of a GaAs layer or a GaInP mixedcrystal layer adjacent with said AlGaAs mixed crystal layer constitutingsaid region of relatively low refractive index with respect to saidsurrounding region not injected with said molecules containing oxygenand located on said identical plane of said part where said ionimplantation of said molecules has been made.
 23. A surface emissionlaser diode according to claim 21, wherein said AlGaAs mixed crystallayer, constituting said region of relatively low refractive index withrespect to said surrounding region not made with ion implantation ofmolecules containing oxygen and located on said identical planeperpendicular to said laser cavity direction, is provided at a locationcorresponding to an anti-node of a standing wave of laser oscillationoccurring in said cavity structure, said AlGaAs mixed crystal layerbeing doped to a higher concentration level as compared with otherAlGaAs mixed crystal layers constituting said surface-emission laserdiode.
 24. A surface-emission laser diode according to claim 21, whereinsaid region of relatively low refractive index is provided in pluralnumbers.
 25. A surface-emission laser diode according to claim 21,wherein said region of relatively low refractive index is providedinside an n-type distributed Bragg reflector constituting one of saidpair of distributed Bragg reflectors.
 26. A surface-emission laser diodeaccording to claim 21, wherein there is further provided a region ofrelatively high refractive index around said region of relatively lowrefractive index provided in spatial overlapping with said laser cavityregion in said laser cavity direction, and wherein there is furtherprovided a cladding region of low refractive index with respect to saidhigh refractive region such that said cladding region is located aroundsaid region of high refractive index.
 27. A surface-emission laser diodeaccording to claim 26, wherein there is provided an anisotropy in awidth of said high refractive region surrounded by said cladding region.28. A surface-emission laser diode according to claim 26, wherein saidcladding region is provided in a pair in a direction perpendicular tosaid laser cavity direction across a laser cavity region.
 29. Asurface-emission laser diode according to claim 21, wherein said regionof relatively low refractive index has an anisotropic shape.
 30. Asurface-emission laser diode according to claim 21, wherein said activelayer comprises a group III-V compound semiconductor material, saidgroup III element comprises at least one of Ga and In, and wherein saidgroup V element comprises one or more of As, N, Sb and P.
 31. Asurface-emission laser diode, comprising: an active layer; a currentconfinement structure defining a current injection region into saidactive layer; and a pair of distributed Bragg reflectors opposing witheach other across said active layer, said current confinement structurecomprising a high resistance region formed by an ion implantationprocess, said surface-emission laser diode comprising an AlGaAs mixedcrystal layer, a part of said AlGaAs mixed crystal layer having aspatial overlapping with said current injection region in a laser cavitydirection, said part of said AlGaAs mixed crystal spatially overlappingwith said current injection region being processed by an ionimplantation process of molecules containing oxygen and a thermalannealing process following said ion implantation process and having arelatively low refractive index as compared with a surrounding regionnot injected with molecules containing oxygen and located on anidentical plane of said part, said plane being perpendicular to saidlaser cavity direction.
 32. A surface-emission laser diode according toclaim 31, wherein there is provided any of a GaAs layer or a GaInP mixedcrystal layer adjacent with said AlGaAs mixed crystal layer constitutingsaid region of relatively low refractive index with respect to saidsurrounding region not injected with said molecules containing oxygenand located on said identical plane of said part where said ionimplantation of said molecules has been made.
 33. A surface emissionlaser diode according to claim 31, wherein said AlGaAs mixed crystallayer, constituting said region of relatively low refractive index withrespect to said surrounding region not made with ion implantation ofmolecules containing oxygen and located on said identical planeperpendicular to said laser cavity direction, is provided at a locationcorresponding to an anti-node of a standing wave of laser oscillationoccurring in said cavity structure, said AlGaAs mixed crystal layerbeing doped to a higher concentration level as compared with otherAlGaAs mixed crystal layers constituting said surface-emission laserdiode.
 34. A surface-emission laser diode according to claim 31, whereinsaid region of relatively low refractive index is provided in pluralnumbers.
 35. A surface-emission laser diode according to claim 31,wherein said region of relatively low refractive index is providedinside an n-type distributed Bragg reflector constituting one of saidpair of distributed Bragg reflectors.
 36. A surface-emission laser diodeaccording to claim 31, wherein there is further provided a region ofrelatively high refractive index around said region of relatively lowrefractive index provided in spatial overlapping with said laser cavityregion in said laser cavity direction, and wherein there is furtherprovided a cladding region of low refractive index with respect to saidhigh refractive region such that said cladding region is located aroundsaid region of high refractive index.
 37. A surface-emission laser diodeaccording to claim 36, wherein there is provided an anisotropy in awidth of said high refractive region surrounded by said cladding region.38. A surface-emission laser diode according to claim 36, wherein saidcladding region is provided in a pair in a direction perpendicular tosaid laser cavity direction across a laser cavity region.
 39. Asurface-emission laser diode according claim 31, wherein said region ofrelatively low refractive index has an anisotropic shape.
 40. Asurface-emission laser diode according to claim 31, wherein said activelayer comprises a group III-V compound semiconductor material, saidgroup III element comprises at least one of Ga and In, and wherein saidgroup V element comprises one or more of As, N, Sb and P.
 41. Asurface-emission laser array comprising a plurality of surface-emissionlaser diodes, said plurality of surface-emission laser diodes beingarranged to form an array, each of said plurality of surface-emissionlaser diodes comprising: an active layer; a pair of cavity spacer layersformed at both sides of said active layer; a current confinementstructure defining a current injection region into said active layer;and a pair of distributed Bragg reflectors opposing with each otheracross a structure formed of said active layer and said cavity spacerlayers, said current confinement structure being formed by a selectiveoxidation process of a semiconductor layer, said pair of distributedBragg reflectors being formed of semiconductor materials, wherein thereis provided a region containing an oxide of Al and having a relativelylow refractive index as compared with a surrounding region in any ofsaid semiconductor distributed Bragg reflector or said cavity spacerlayer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 42. Asurface-emission laser array comprising a plurality of surface-emissionlaser diodes, said plurality of surface-emission laser diodes beingarranged to form an array, each of said plurality of surface-emissionlaser diodes comprising: an active layer; a pair of cavity spacer layersformed at both sides of said active layer; a current confinementstructure defining a current injection region into said active layer;and a pair of distributed Bragg reflectors opposing with each otheracross a structure formed of said active layer and said pair of cavityspacer layers, said current confinement structure comprising a highresistance region formed by an ion implantation process, said pair ofdistributed Bragg reflectors being formed of semiconductor materials,wherein there is provided a region containing an oxide of Al and havinga relatively low refractive index than a surrounding region in any ofsaid semiconductor distributed Bragg reflector or said cavity spacerlayer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 43. Asurface-emission laser array comprising a plurality of surface-emissionlaser diodes, said plurality of surface-emission laser diodes beingarranged to form an array, each of said plurality of surface-emissionlaser diode comprising: an active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across saidactive layer; said current confinement structure being formed of aselective oxidation process of a semiconductor material, saidsurface-emission laser diode comprising an AlGaAs mixed crystal layer, apart of said AlGaAs mixed crystal layer having spatial overlapping withsaid current injection region in a laser cavity direction, said part ofsaid AlGaAs mixed crystal being processed by selective ion implantationof molecules containing oxygen and a thermal annealing process followingsaid selective ion implantation process, said part of said AlGaAs mixedcrystal having a relatively low refractive index as compared with asurrounding region wherein ion implantation of molecules containingoxygen is not made and located on an identical plane of said part, saidplane being perpendicular to said laser cavity direction.
 44. Asurface-emission laser array comprising a plurality of surface-emissionlaser diodes, said plurality of surface-emission laser diodes beingarranged to form an array, each of said plurality of surface-emissionlaser diode comprising: an active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across saidactive layer, said current confinement structure comprising a highresistance region formed by an ion implantation process, saidsurface-emission laser diode comprising an AlGaAs mixed crystal layer, apart of said AlGaAs mixed crystal layer having a spatial overlappingwith said current injection region in a laser cavity direction, saidpart of said AlGaAs mixed crystal spatially overlapping with saidcurrent injection region being processed by an ion implantation processof molecules containing oxygen and a thermal annealing process followingsaid ion implantation process and having a relatively low refractiveindex as compared with a surrounding region not injected with moleculescontaining oxygen and located on an identical plane of said part, saidplane being perpendicular to said laser cavity direction.
 45. An opticalinterconnection system having an optical source comprising asurface-emission laser diode as an optical source, said surface-emissionlaser diode comprising: an active layer; a pair of cavity spacer layersformed at both sides of said active layer; a current confinementstructure defining a current injection region into said active layer;and a pair of distributed Bragg reflectors opposing with each otheracross a structure formed of said active layer and said cavity spacerlayers, said current confinement structure being formed by a selectiveoxidation process of a semiconductor layer, said pair of distributedBragg reflectors being formed of semiconductor materials, wherein thereis provided a region containing an oxide of Al and having a relativelylow refractive index as compared with a surrounding region in any ofsaid semiconductor distributed Bragg reflector or said cavity spacerlayer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 46. An opticalinterconnection system having an optical source comprising asurface-emission laser diode as an optical source, said surface-emissionlaser diode comprising: an active layer; a pair of cavity spacer layersformed at both sides of said active layer; a current confinementstructure defining a current injection region into said active layer;and a pair of distributed Bragg reflectors opposing with each otheracross a structure formed of said active layer and said pair of cavityspacer layers, said current confinement structure comprising a highresistance region formed by an ion implantation process, said pair ofdistributed Bragg reflectors being formed of semiconductor materials,wherein there is provided a region containing an oxide of Al and havinga relatively low refractive index than a surrounding region in any ofsaid semiconductor distributed Bragg reflector or said cavity spacerlayer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 47. An opticalinterconnection system having an optical source comprising asurface-emission laser diode as an optical source, said surface-emissionlaser diode comprising: an active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across saidactive layer; said current confinement structure being formed of aselective oxidation process of a semiconductor material, saidsurface-emission laser diode comprising an AlGaAs mixed crystal layer, apart of said AlGaAs mixed crystal layer having spatial overlapping withsaid current injection region in a laser cavity direction, said part ofsaid AlGaAs mixed crystal being processed by selective ion implantationof molecules containing oxygen and a thermal annealing process followingsaid selective ion implantation process, said part of said AlGaAs mixedcrystal having a relatively low refractive index as compared with asurrounding region wherein ion implantation of molecules containingoxygen is not made and located on an identical plane of said part, saidplane being perpendicular to said laser cavity direction.
 48. An opticalinterconnection system having an optical source comprising asurface-emission laser diode as an optical source, said surface-emissionlaser diode comprising: an active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across saidactive layer, said current confinement structure comprising a highresistance region formed by an ion implantation process, saidsurface-emission laser diode comprising an AlGaAs mixed crystal layer, apart of said AlGaAs mixed crystal layer having a spatial overlappingwith said current injection region in a laser cavity direction, saidpart of said AlGaAs mixed crystal spatially overlapping with saidcurrent injection region being processed by an ion implantation processof molecules containing oxygen and a thermal annealing process followingsaid ion implantation process and having a relatively low refractiveindex as compared with a surrounding region not injected with moleculescontaining oxygen and located on an identical plane of said part, saidplane being perpendicular to said laser cavity direction.
 49. An opticalinterconnection system having an optical source comprising asurface-emission laser array, said surface-emission laser arraycomprising a plurality of surface-emission laser diodes, said pluralityof surface-emission laser diodes being arranged to form an array, eachof said plurality of surface-emission laser diodes comprising: an activelayer; a pair of cavity spacer layers formed at both sides of saidactive layer; a current confinement structure defining a currentinjection region into said active layer; and a pair of distributed Braggreflectors opposing with each other across a structure formed of saidactive layer and said cavity spacer layers, said current confinementstructure being formed by a selective oxidation process of asemiconductor layer, said pair of distributed Bragg reflectors beingformed of semiconductor materials, wherein there is provided a regioncontaining an oxide of Al and having a relatively low refractive indexas compared with a surrounding region in any of said semiconductordistributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 50. An opticalinterconnection system having an optical source comprising asurface-emission laser array, said surface-emission laser arraycomprising a plurality of surface-emission laser diodes, said pluralityof surface-emission laser diodes being arranged to form an array, eachof said plurality of surface-emission laser diodes comprising: an activelayer; a pair of cavity spacer layers formed at both sides of saidactive layer; a current confinement structure defining a currentinjection region into said active layer; and a pair of distributed Braggreflectors opposing with each other across a structure formed of saidactive layer and said cavity spacer layers, said current confinementstructure comprising a high resistance region formed by an ionimplantation process, said pair of distributed Bragg reflectors beingformed of semiconductor materials, wherein there is provided a regioncontaining an oxide of Al and having a relatively low refractive indexas compared with a surrounding region in any of said semiconductordistributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 51. An opticalcommunication system that uses a surface-emission laser diode as acommunication optical source, said surface-emission laser diodecomprising: an active layer; a pair of cavity spacer layers formed atboth sides of said active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across astructure formed of said active layer and said cavity spacer layers,said current confinement structure being formed by a selective oxidationprocess of a semiconductor layer, said pair of distributed Braggreflectors being formed of semiconductor materials, wherein there isprovided a region containing an oxide of Al and having a relatively lowrefractive index as compared with a surrounding region in any of saidsemiconductor distributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 52. An opticalcommunication system that uses a surface-emission laser diode as acommunication optical source, said surface-emission laser diodecomprising: an active layer; a pair of cavity spacer layers formed atboth sides of said active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across astructure formed of said active layer and said pair of cavity spacerlayers, said current confinement structure comprising a high resistanceregion formed by an ion implantation process, said pair of distributedBragg reflectors being formed of semiconductor materials, wherein thereis provided a region containing an oxide of Al and having a relativelylow refractive index than a surrounding region in any of saidsemiconductor distributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 53. An opticalcommunication system that uses a surface-emission laser diode as acommunication optical source, said surface-emission laser diodecomprising: an active layer; a current confinement structure defining acurrent injection region into said active layer; and a pair ofdistributed Bragg reflectors opposing with each other across said activelayer; said current confinement structure being formed of a selectiveoxidation process of a semiconductor material, said surface-emissionlaser diode comprising an AlGaAs mixed crystal layer, a part of saidAlGaAs mixed crystal layer having spatial overlapping with said currentinjection region in a laser cavity direction, said part of said AlGaAsmixed crystal being processed by selective ion implantation of moleculescontaining oxygen and a thermal annealing process following saidselective ion implantation process, said part of said AlGaAs mixedcrystal having a relatively low refractive index as compared with asurrounding region wherein ion implantation of molecules containingoxygen is not made and located on an identical plane of said part, saidplane being perpendicular to said laser cavity direction.
 54. An opticalcommunication system that uses a surface-emission laser diode as acommunication optical source, said surface-emission laser diode,comprising: an active layer; a current confinement structure defining acurrent injection region into said active layer; and a pair ofdistributed Bragg reflectors opposing with each other across said activelayer, said current confinement structure comprising a high resistanceregion formed by an ion implantation process, said surface-emissionlaser diode comprising an AlGaAs mixed crystal layer, a part of saidAlGaAs mixed crystal layer having a spatial overlapping with saidcurrent injection region in a laser cavity direction, said part of saidAlGaAs mixed crystal spatially overlapping with said current injectionregion being processed by an ion implantation process of moleculescontaining oxygen and a thermal annealing process following said ionimplantation process and having a relatively low refractive index ascompared with a surrounding region not injected with moleculescontaining oxygen and located on an identical plane of said part, saidplane being perpendicular to said laser cavity direction.
 55. An opticalcommunication system that uses a surface-emission laser array as acommunication optical source, said surface-emission laser arraycomprising a plurality of surface-emission laser diodes, said pluralityof surface-emission laser diodes being arranged to form an array, eachof said plurality of surface-emission laser diodes comprising: an activelayer; a pair of cavity spacer layers formed at both sides of saidactive layer; a current confinement structure defining a currentinjection region into said active layer; and a pair of distributed Braggreflectors opposing with each other across said active layer and saidcavity spacer layers, said current confinement structure being formed bya selective oxidation process of a semiconductor layer, said pair ofdistributed Bragg reflectors being formed of semiconductor materials,wherein there is provided a region containing an oxide of Al and havinga relatively low refractive index as compared with a surrounding regionin any of said semiconductor distributed Bragg reflector or said cavityspacer layer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 56. An opticalcommunication system that uses a surface-emission laser array as acommunication optical source, said surface-emission laser arraycomprising a plurality of surface-emission laser diodes, said pluralityof surface-emission laser diodes being arranged to form an array, eachof said plurality of surface-emission laser diodes comprising: an activelayer; a pair of cavity spacer layers formed at both sides of saidactive layer; a current confinement structure defining a currentinjection region into said active layer; and a pair of distributed Braggreflectors opposing with each other across said active layer and saidcavity spacer layers, said current confinement structure comprising ahigh resistance region formed by an ion implantation process, said pairof distributed Bragg reflectors being formed of semiconductor materials,wherein there is provided a region containing an oxide of Al and havinga relatively low refractive index as compared with a surrounding regionin any of said semiconductor distributed Bragg reflector or said cavityspacer layer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 57. Anelectro-photographic system that uses a surface-emission laser diode asa writing optical source, said A surface-emission laser diodecomprising: an active layer; a pair of cavity spacer layers formed atboth sides of said active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across astructure formed of said active layer and said cavity spacer layers,said current confinement structure being formed by a selective oxidationprocess of a semiconductor layer, said pair of distributed Braggreflectors being formed of semiconductor materials, wherein there isprovided a region containing an oxide of Al and having a relatively lowrefractive index as compared with a surrounding region in any of saidsemiconductor distributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 58. Anelectro-photographic system using a surface-emission laser diode as awriting optical source, said surface-emission laser diode comprising: anactive layer; a pair of cavity spacer layers formed at both sides ofsaid active layer; a current confinement structure defining a currentinjection region into said active layer; and a pair of distributed Braggreflectors opposing with each other across a structure formed of saidactive layer and said pair of cavity spacer layers, said currentconfinement structure comprising a high resistance region formed by anion implantation process, said pair of distributed Bragg reflectorsbeing formed of semiconductor materials, wherein there is provided aregion containing an oxide of Al and having a relatively low refractiveindex than a surrounding region in any of said semiconductor distributedBragg reflector or said cavity spacer layer in correspondence to a partspatially overlapping with said current injection region in a lasercavity direction.
 59. An electro-photographic system using asurface-emission laser diode as a writing optical source, saidsurface-emission laser diode comprising: an active layer; a currentconfinement structure defining a current injection region into saidactive layer; and a pair of distributed Bragg reflectors opposing witheach other across said active layer; said current confinement structurebeing formed of a selective oxidation process of a semiconductormaterial, said surface-emission laser diode comprising an AlGaAs mixedcrystal layer, a part of said AlGaAs mixed crystal layer having spatialoverlapping with said current injection region in a laser cavitydirection, said part of said AlGaAs mixed crystal being formed byselective ion implantation of molecules containing oxygen and a thermalannealing process following said selective ion implantation process,said part of said AlGaAs mixed crystal having a relatively lowrefractive index as compared with a surrounding region wherein ionimplantation of molecules containing oxygen is not made and located onan identical plane of said part, said plane being perpendicular to saidlaser cavity direction.
 60. An electro-photographic system using asurface-emission laser diode as a writing optical source, saidsurface-emission laser diode, comprising: an active layer; a currentconfinement structure defining a current injection region into saidactive layer; and a pair of distributed Bragg reflectors opposing witheach other across said active layer, said current confinement structurecomprising a high resistance region formed by an ion implantationprocess, said surface-emission laser diode comprising an AlGaAs mixedcrystal layer, a part of said AlGaAs mixed crystal layer having aspatial overlapping with said current injection region in a laser cavitydirection, said part of said AlGaAs mixed crystal spatially overlappingwith said current injection region being processed by an ionimplantation process of molecules containing oxygen and a thermalannealing process following said ion implantation process and having arelatively low refractive index as compared with a surrounding regionnot injected with molecules containing oxygen and located on anidentical plane of said part, said plane being perpendicular to saidlaser cavity direction.
 61. An electro-photographic system using asurface-emission laser array as a writing optical source, saidsurface-emission laser array comprising a plurality of surface-emissionlaser diodes, said plurality of surface-emission laser diodes beingarranged to form an array, each of said plurality of surface-emissionlaser diodes comprising: an active layer; a pair of cavity spacer layersformed at both sides of said active layer; a current confinementstructure defining a current injection region into said active layer;and a pair of distributed Bragg reflectors opposing with each otheracross a structure formed of said active layer and said cavity spacerlayers, said current confinement structure being formed by a selectiveoxidation process of a semiconductor layer, said pair of distributedBragg reflectors being formed of semiconductor materials, wherein thereis provided a region containing an oxide of Al and having a relativelylow refractive index as compared with a surrounding region in any ofsaid semiconductor distributed Bragg reflector or said cavity spacerlayer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 62. Anelectro-photographic system using a surface-emission laser array as awriting optical source, said surface-emission laser array comprising aplurality of surface-emission laser diodes, said plurality ofsurface-emission laser diodes being arranged to form an array, each ofsaid plurality of surface-emission laser diodes comprising: an activelayer; a pair of cavity spacer layers formed at both sides of saidactive layer; a current confinement structure defining a currentinjection region into said active layer; and a pair of distributed Braggreflectors opposing with each other across a structure formed of saidactive layer and said cavity spacer layers, said current confinementstructure comprising a high resistance region formed by an ionimplantation process, said pair of distributed Bragg reflectors beingformed of semiconductor materials, wherein there is provided a regioncontaining an oxide of Al and having a relatively low refractive indexas compared with a surrounding region in any of said semiconductordistributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 63. An optical disk systemthat uses a surface-emission laser diode as a reading/writing opticalsource, said surface-emission laser diode comprising: an active layer; apair of cavity spacer layers formed at both sides of said active layer;a current confinement structure defining a current injection region intosaid active layer; and a pair of distributed Bragg reflectors opposingwith each other across a structure formed of said active layer and saidcavity spacer layers, said current confinement structure being formed bya selective oxidation process of a semiconductor layer, said pair ofdistributed Bragg reflectors being formed of semiconductor materials,wherein there is provided a region containing an oxide of Al and havinga relatively low refractive index as compared with a surrounding regionin any of said semiconductor distributed Bragg reflector or said cavityspacer layer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 64. An opticaldisk system that uses a surface-emission laser diode as areading/writing optical source, said surface-emission laser diodecomprising: an active layer; a pair of cavity spacer layers formed atboth sides of said active layer; a current confinement structuredefining a current injection region into said active layer; and a pairof distributed Bragg reflectors opposing with each other across astructure formed of said active layer and said pair of cavity spacerlayers, said current confinement structure comprising a high resistanceregion formed by an ion implantation process, said pair of distributedBragg reflectors being formed of semiconductor materials, wherein thereis provided a region containing an oxide of Al and having a relativelylow refractive index than a surrounding region in any of saidsemiconductor distributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 65. An optical disk systemthat uses a surface-emission laser diode as a reading/writing opticalsource, said surface-emission laser diode comprising: an active layer; acurrent confinement structure defining a current injection region intosaid active layer; and a pair of distributed Bragg reflectors opposingwith each other across said active layer; said current confinementstructure being formed of a selective oxidation process of asemiconductor material, said surface-emission laser diode comprising anAlGaAs mixed crystal layer, a part of said AlGaAs mixed crystal layerhaving spatial overlapping with said current injection region in a lasercavity direction, said part of said AlGaAs mixed crystal being formed byselective ion implantation of molecules containing oxygen and a thermalannealing process following said selective ion implantation process,said part of said AlGaAs mixed crystal having a relatively lowrefractive index as compared with a surrounding region wherein ionimplantation of molecules containing oxygen is not made and located onan identical plane of said part, said plane being perpendicular to saidlaser cavity direction.
 66. An optical disk system that uses asurface-emission laser diode as a reading/writing optical source, saidsurface-emission laser diode, comprising: an active layer; a currentconfinement structure defining a current injection region into saidactive layer; and a pair of distributed Bragg reflectors opposing witheach other across said active layer, said current confinement structurecomprising a high resistance region formed by an ion implantationprocess, said surface-emission laser diode comprising an AlGaAs mixedcrystal layer, a part of said AlGaAs mixed crystal layer having aspatial overlapping with said current injection region in a laser cavitydirection, said part of said AlGaAs mixed crystal spatially overlappingwith said current injection region being processed by an ionimplantation process of molecules containing oxygen and a thermalannealing process following said ion implantation process and having arelatively low refractive index as compared with a surrounding regionnot injected with molecules containing oxygen and located on anidentical plane of said part, said plane being perpendicular to saidlaser cavity direction.
 67. An optical disk system that uses asurface-emission laser array as a reading/writing optical source, saidsurface-emission laser array comprising a plurality of surface-emissionlaser diodes, said plurality of surface-emission laser diodes beingarranged to form an array, each of said plurality of surface-emissionlaser diodes comprising: an active layer; a pair of cavity spacer layersformed at both sides of said active layer; a current confinementstructure defining a current injection region into said active layer;and a pair of distributed Bragg reflectors opposing with each otheracross a structure formed of said active layer and said cavity spacerlayers, said current confinement structure being formed by a selectiveoxidation process of a semiconductor layer, said pair of distributedBragg reflectors being formed of semiconductor materials, wherein thereis provided a region containing an oxide of Al and having a relativelylow refractive index as compared with a surrounding region in any ofsaid semiconductor distributed Bragg reflector or said cavity spacerlayer in correspondence to a part spatially overlapping with saidcurrent injection region in a laser cavity direction.
 68. An opticaldisk system that uses a surface-emission laser array as areading/writing optical source, said surface-emission laser arraycomprising a plurality of surface-emission laser diodes, said pluralityof surface-emission laser diodes being arranged to form an array, eachof said plurality of surface-emission laser diodes comprising: an activelayer; a pair of cavity spacer layers formed at both sides of saidactive layer; a current confinement structure defining a currentinjection region into said active layer; and a pair of distributed Braggreflectors opposing with each other across a structure formed of saidactive layer and said cavity spacer layers, said current confinementstructure comprising a high resistance region formed by an ionimplantation process, said pair of distributed Bragg reflectors beingformed of semiconductor materials, wherein there is provided a regioncontaining an oxide of Al and having a relatively low refractive indexas compared with a surrounding region in any of said semiconductordistributed Bragg reflector or said cavity spacer layer incorrespondence to a part spatially overlapping with said currentinjection region in a laser cavity direction.
 69. A surface-emissionlaser diode constructed on a substrate having a substrate surface,comprising: an active layer parallel to said substrate surface; a pairof cavity spacer layers provided so as to sandwich said active layer; apair of distributed Bragg reflectors provided across said active layerand said cavity spacer layers so as to sandwich said active layer andsaid cavity spacer layers therebetween; a high resistance regiondefining a current injection region for injecting a current into saidactive layer, laser oscillation being caused in a laser cavity regionbetween said pair of distributed Bragg reflectors acting as cavitymirrors in a direction perpendicular to said substrate surface incorrespondence to said current injection region, a refractive indexstructure provided in a plane parallel to said substrate surface, saidrefractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein any of a width of said lowrefractive index core or a shape of said periodic structure is changedbetween a specific direction parallel to said substrate surface andother directions parallel to said substrate surface different from saidspecific direction.
 70. A surface-emission laser diode according toclaim 69, wherein said low refractive index core has a width differentbetween two directions parallel to said substrate surface and crossingperpendicularly with each other.
 71. A surface-emission layer diodeaccording to any of claims 69, wherein a reflection wavelength band ofsaid periodic structure is set, in one of said two directions crossingperpendicularly with each other, to be longer than a wavelength of afundamental transverse mode in said same direction and projected uponsaid substrate surface.
 72. A surface-emission laser diode according toclaim 69, wherein said periodic structure is different between twodirections parallel to said substrate surface and crossingperpendicularly with each other.
 73. A surface-emission layer diodeaccording to any of claims 72, wherein a reflection wavelength band ofsaid periodic structure is set, in one of said two directions crossingperpendicularly with each other, to be longer than a wavelength of afundamental transverse mode in said same direction and projected uponsaid substrate surface.
 74. A surface-emission laser diode according toclaim 65, wherein said active layer is formed of a III-V semiconductormaterial, said active layer containing any or both of Ga and In as agroup III element, said active layer containing any or all of As, N, Sband P as a group V element.
 75. A surface-emission laser diodeconstructed on a substrate having a substrate surface, comprising: anactive layer parallel to said substrate surface; a pair of cavity spacerlayers provided so as to sandwich said active layer; a pair ofdistributed Bragg reflectors provided across said active layer and saidcavity spacer layers so as to sandwich said active layer and said cavityspacer layers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein said periodic structure is provided partially in saidplane parallel to said substrate surface in a specific direction.
 76. Asurface-emission laser diode according to claim 75, wherein said activelayer is formed of a III-V semiconductor material, said active layercontaining any or both of Ga and In as a group III element, said activelayer containing any or all of As, N, Sb and P as a group V element. 77.A surface-emission laser array formed of a plurality of surface-emissionlaser diodes, each of said plurality of surface-emission laser diodesbeing constructed on a substrate having a substrate surface andcomprising: an active layer parallel to said substrate surface; a pairof cavity spacer layers provided so as to sandwich said active layer; apair of distributed Bragg reflectors provided across said active layerand said cavity spacer layers so as to sandwich said active layer andsaid cavity spacer layers therebetween; a high resistance regiondefining a current injection region for injecting a current into saidactive layer, laser oscillation being caused in a laser cavity regionbetween said pair of distributed Bragg reflectors acting as cavitymirrors in a direction perpendicular to said substrate surface incorrespondence to said current injection region, a refractive indexstructure provided in a plane parallel to said substrate surface, saidrefractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein any of a width of said lowrefractive index core or a shape of said periodic structure is changedbetween a specific direction parallel to said substrate surface andother directions parallel to said substrate surface different from saidspecific direction.
 78. A surface-emission laser array formed of aplurality of surface-emission laser diodes, each of said plurality ofsurface-emission laser diodes being constructed on a substrate having asubstrate surface and comprising: an active layer parallel to saidsubstrate surface; a pair of cavity spacer layers provided so as tosandwich said active layer; a pair of distributed Bragg reflectorsprovided across said active layer and said cavity spacer layers so as tosandwich said active layer and said cavity spacer layers therebetween; ahigh resistance region defining a current injection region for injectinga current into said active layer, laser oscillation being caused in alaser cavity region between said pair of distributed Bragg reflectorsacting as cavity mirrors in a direction perpendicular to said substratesurface in correspondence to said current injection region, a refractiveindex structure provided in a plane parallel to said substrate surface,said refractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein said periodic structure isprovided partially in said plane parallel to said substrate surface in aspecific direction.
 79. An optical interconnection system that uses thesurface-emission laser diode constructed on a substrate having asubstrate surface, said surface-emission laser diode comprising: anactive layer parallel to said substrate surface; a pair of cavity spacerlayers provided so as to sandwich said active layer; a pair ofdistributed Bragg reflectors provided across said active layer and saidcavity spacer layers so as to sandwich said active layer and said cavityspacer layers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein any of a width of said low refractive index core or ashape of said periodic structure is changed between a specific directionparallel to said substrate surface and other directions parallel to saidsubstrate surface different from said specific direction.
 80. An opticalinterconnection system that uses the surface-emission laser diodeconstructed on a substrate having a substrate surface, saidsurface-emission laser diode comprising: an active layer parallel tosaid substrate surface; a pair of cavity spacer layers provided so as tosandwich said active layer; a pair of distributed Bragg reflectorsprovided across said active layer and said cavity spacer layers so as tosandwich said active layer and said cavity spacer layers therebetween; ahigh resistance region defining a current injection region for injectinga current into said active layer, laser oscillation being caused in alaser cavity region between said pair of distributed Bragg reflectorsacting as cavity mirrors in a direction perpendicular to said substratesurface in correspondence to said current injection region, a refractiveindex structure provided in a plane parallel to said substrate surface,said refractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein said periodic structure isprovided partially in said plane parallel to said substrate surface in aspecific direction.
 81. An optical interconnection system that uses thesurface-emission laser array formed of a plurality of surface-emissionlaser diodes, each of said plurality of surface-emission laser diodesbeing constructed on a substrate having a substrate surface andcomprising: an active layer parallel to said substrate surface; a pairof cavity spacer layers provided so as to sandwich said active layer; apair of distributed Bragg reflectors provided across said active layerand said cavity spacer layers so as to sandwich said active layer andsaid cavity spacer layers therebetween; a high resistance regiondefining a current injection region for injecting a current into saidactive layer, laser oscillation being caused in a laser cavity regionbetween said pair of distributed Bragg reflectors acting as cavitymirrors in a direction perpendicular to said substrate surface incorrespondence to said current injection region, a refractive indexstructure provided in a plane parallel to said substrate surface, saidrefractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein any of a width of said lowrefractive index core or a shape of said periodic structure is changedbetween a specific direction parallel to said substrate surface andother directions parallel to said substrate surface different from saidspecific direction.
 82. An optical communication system that uses thesurface-emission laser diode constructed on a substrate having asubstrate surface, said surface-emission laser diode comprising: anactive layer parallel to said substrate surface; a pair of cavity spacerlayers provided so as to sandwich said active layer; a pair ofdistributed Bragg reflectors provided across said active layer and saidcavity spacer layers so as to sandwich said active layer and said cavityspacer layers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein any of a width of said low refractive index core or ashape of said periodic structure is changed between a specific directionparallel to said substrate surface and other directions parallel to saidsubstrate surface different from said specific direction.
 83. An opticalcommunication system that uses the surface-emission laser diodeconstructed on a substrate having a substrate surface, saidsurface-emission laser diode comprising: an active layer parallel tosaid substrate surface; a pair of cavity spacer layers provided so as tosandwich said active layer; a pair of distributed Bragg reflectorsprovided across said active layer and said cavity spacer layers so as tosandwich said active layer and said cavity spacer layers therebetween; ahigh resistance region defining a current injection region for injectinga current into said active layer, laser oscillation being caused in alaser cavity region between said pair of distributed Bragg reflectorsacting as cavity mirrors in a direction perpendicular to said substratesurface in correspondence to said current injection region, a refractiveindex structure provided in a plane parallel to said substrate surface,said refractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein said periodic structure isprovided partially in said plane parallel to said substrate surface in aspecific direction.
 84. An optical communication system that uses asurface-emission laser array formed of a plurality of surface-emissionlaser diodes, each of said plurality of surface-emission laser diodesbeing constructed on a substrate having a substrate surface andcomprising: an active layer parallel to said substrate surface; a pairof cavity spacer layers provided so as to sandwich said active layer; apair of distributed Bragg reflectors provided across said active layerand said cavity spacer layers so as to sandwich said active layer andsaid cavity spacer layers therebetween; a high resistance regiondefining a current injection region for injecting a current into saidactive layer, laser oscillation being caused in a laser cavity regionbetween said pair of distributed Bragg reflectors acting as cavitymirrors in a direction perpendicular to said substrate surface incorrespondence to said current injection region, a refractive indexstructure provided in a plane parallel to said substrate surface, saidrefractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein any of a width of said lowrefractive index core or a shape of said periodic structure is changedbetween a specific direction parallel to said substrate surface andother directions parallel to said substrate surface different from saidspecific direction.
 85. An electrophotographic system that uses thesurface-emission laser diode constructed on a substrate having asubstrate surface, said surface-emission laser diode comprising: anactive layer parallel to said substrate surface; a pair of cavity spacerlayers provided so as to sandwich said active layer; a pair ofdistributed Bragg reflectors provided across said active layer and saidcavity spacer layers so as to sandwich said active layer and said cavityspacer layers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein any of a width of said low refractive index core or ashape of said periodic structure is changed between a specific directionparallel to said substrate surface and other directions parallel to saidsubstrate surface different from said specific direction.
 86. Anelectrophotographic system that uses the surface-emission laser diodeconstructed on a substrate having a substrate surface, saidsurface-emission laser diode comprising: an active layer parallel tosaid substrate surface; a pair of cavity spacer layers provided so as tosandwich said active layer; a pair of distributed Bragg reflectorsprovided across said active layer and said cavity spacer layers so as tosandwich said active layer and said cavity spacer layers therebetween; ahigh resistance region defining a current injection region for injectinga current into said active layer, laser oscillation being caused in alaser cavity region between said pair of distributed Bragg reflectorsacting as cavity mirrors in a direction perpendicular to said substratesurface in correspondence to said current injection region, a refractiveindex structure provided in a plane parallel to said substrate surface,said refractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein said periodic structure isprovided partially in said plane parallel to said substrate surface in aspecific direction.
 87. An electrophotographic system that uses asurface-emission laser array formed of a plurality of surface-emissionlaser diodes, each of said plurality of surface-emission laser diodesbeing constructed on a substrate having a substrate surface andcomprising: an active layer parallel to said substrate surface; a pairof cavity spacer layers provided so as to sandwich said active layer; apair of distributed Bragg reflectors provided across said active layerand said cavity spacer layers so as to sandwich said active layer andsaid cavity spacer layers therebetween; a high resistance regiondefining a current injection region for injecting a current into saidactive layer, laser oscillation being caused in a laser cavity regionbetween said pair of distributed Bragg reflectors acting as cavitymirrors in a direction perpendicular to said substrate surface incorrespondence to said current injection region, a refractive indexstructure provided in a plane parallel to said substrate surface, saidrefractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surroundingsaid-low refractive index core and a high refractive index regionsurrounding said low refractive index region, said low refractive indexregion and said high refractive index region being repeated alternatelyin said plane parallel to said substrate, wherein any of a width of saidlow refractive index core or a shape of said periodic structure ischanged between a specific direction parallel to said substrate surfaceand other directions parallel to said substrate surface different fromsaid specific direction.
 88. An optical disk system that uses a surfaceemission laser constructed on a substrate having a substrate surface,said surface-emission laser diode comprising: an active layer parallelto said substrate surface; a pair of cavity spacer layers provided so asto sandwich said active layer; a pair of distributed Bragg reflectorsprovided across said active layer and said cavity spacer layers so as tosandwich said active layer and said cavity spacer layers therebetween; ahigh resistance region defining a current injection region for injectinga current into said active layer, laser oscillation being caused in alaser cavity region between said pair of distributed Bragg reflectorsacting as cavity mirrors in a direction perpendicular to said substratesurface in correspondence to said current injection region, a refractiveindex structure provided in a plane parallel to said substrate surface,said refractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein any of a width of said lowrefractive index core or a shape of said periodic structure is changedbetween a specific direction parallel to said substrate surface andother directions parallel to said substrate surface different from saidspecific direction.
 89. An optical disk system that uses a surfaceemission laser diode constructed on a substrate having a substratesurface, said surface-emission laser diode comprising: an active layerparallel to said substrate surface; a pair of cavity spacer layersprovided so as to sandwich said active layer; a pair of distributedBragg reflectors provided across said active layer and said cavityspacer layers so as to sandwich said active layer and said cavity spacerlayers therebetween; a high resistance region defining a currentinjection region for injecting a current into said active layer, laseroscillation being caused in a laser cavity region between said pair ofdistributed Bragg reflectors acting as cavity mirrors in a directionperpendicular to said substrate surface in correspondence to saidcurrent injection region, a refractive index structure provided in aplane parallel to said substrate surface, said refractive indexstructure comprising: a low refractive index core including said lasercavity region at a central part thereof; and a periodic structureprovided around said low refractive index core, said periodic structurecomprising a low refractive region surrounding said low refractive indexcore and a high refractive index region surrounding said low refractiveindex region, said low refractive index region and said high refractiveindex region being repeated alternately in said plane parallel to saidsubstrate, wherein said periodic structure is provided partially in saidplane parallel to said substrate surface in a specific direction.
 90. Anoptical disk system that uses a surface-emission laser array formed of aplurality of surface-emission laser diodes, each of said plurality ofsurface-emission laser diodes being constructed on a substrate having asubstrate surface and comprising: an active layer parallel to saidsubstrate surface; a pair of cavity spacer layers provided so as tosandwich said active layer; a pair of distributed Bragg reflectorsprovided across said active layer and said cavity spacer layers so as tosandwich said active layer and said cavity spacer layers therebetween; ahigh resistance region defining a current injection region for injectinga current into said active layer, laser oscillation being caused in alaser cavity region between said pair of distributed Bragg reflectorsacting as cavity mirrors in a direction perpendicular to said substratesurface in correspondence to said current injection region, a refractiveindex structure provided in a plane parallel to said substrate surface,said refractive index structure comprising: a low refractive index coreincluding said laser cavity region at a central part thereof; and aperiodic structure provided around said low refractive index core, saidperiodic structure comprising a low refractive region surrounding saidlow refractive index core and a high refractive index region surroundingsaid low refractive index region, said low refractive index region andsaid high refractive index region being repeated alternately in saidplane parallel to said substrate, wherein any of a width of said lowrefractive index core or a shape of said periodic structure is changedbetween a specific direction parallel to said substrate surface andother directions parallel to said substrate surface different from saidspecific direction.